’t 


THE  UNIVERSITY 
OF  ILLINOIS 
LIBRARY 


575 

L76r5 


I 


Alfred  Russel  Wallace,  in  the  Contemporary  Review  ol 
August,  1908,  p.  140,  in  an  article  on  ‘The  Present  Posi- 
tion of  Darwinism,'  writes  as  follows : 

* In  conclusion,  I would  suggest  to  those  of  my  readers 
who  are  interested  in  the  great  questions  associated  with  the 
name  of  Darwin,  but  who  have  not  had  the  means  of  study- 
ing the  facts  either  in  the  field  or  the  library,  that  in  order 
to  obtain  some  real  comprehension  of  the  issue  involved  in 
the  controversy  now  going  on  they  should  read  at  least  one 
book  on  each  side.  The  first  I would  recommend  is  a 
volume  by  Mr.  R.  H.  Lock  on  “Variation,  Heredity  and 
Evolution”  (1906)  as  the  only  recent  book  giving  an  account 
of  the  whole  subject  from  the  point  of  view  of  the  Mendelians 
and  Mutationists.  When  they  have  mastered  this,  I ask 
them  to  read  my  own  book  on  “ Darwinism”  (1901),  which, 
though  published  before  Mendelism  became  prominent, 
gives  some  idea  in  popular  language  of  the  vast  range  of 
subjects  which  Darwinism  explains,  and  adduces  a sufficient 
body  of  facts  to  show  the  inadequacy  of  the  whole  series  of 
phenomena  yet  made  public. 

‘ Having  read  these  two  works  and  again  considered  the 
_ arguments  adduced  in  this  article,  I leave  them  to  form 
their  conclusions  as  to  whether  Darwinism  is  or  is  not  an 
“ unsuccessful  hypothesis.” ' 


‘Alfred  R.  Wallace.’ 


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in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


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VARIATION,  HEREDITY,  AND 
EVOLUTION 


f 


\ 


THE  LIBRARY 
OF  THE 

UHIYERSITY  OF  ILLINOIS 


{Frontispiece, 


RECENT  PROGRESS  IN 
THE  STUDY  OF  VARIATION, 
HEREDITY,  AND  EVOLUTION 

BY 

ROBERT  HEATH  LOCK,  M.A.,  Sc.D. 

SOMETIME  FELLOW  OF  GONVILLE  AND  CAIUS  COLLEGE,  CAMBRIDGE  ; 

LATE  ASSISTANT  DIRECTOR  OF  THE  ROYAL  BOTANIC 
GARDENS,  CEYLON 

REVISED  BY 

LEONARD  DONCASTER,  Sc.D.,  F.R.S. 

PROFESSOR  OF  ZOOLOGY,  UNIVERSITY  OF  LIVERPOO!. 


BIOGRAPHICAL  NOTE  BY  BELLA  SIDNEY  WOOLF 

(MRS.  R.  H,  LOCK) 


NEW  YORK 

E.  P.  DUTTON  AND  COMPANY 
1920 


Imr3T  Kdition,  December^  1006 
Keprinied,  April  y 1907 

,,  Marchy  1909 

Second  Edition,  Jmtey  1909 
Third  Edition,  September y 1911 
Fourth  Edition,  Jidy^  1916 
Fifth  Edition,  ApnU  1Q20 


Printed  in  Great  Britain 


0 


\ 


5n( 


PREFACE  TO  THE  FOURTH  EDITION 


A CERTAIN  number  of  additions  have  been  made  since 
the  first  edition  was  published ; these  were  enumerated 
by  my  husband  in  the  prefaces  to  the  second  and  third 
editions  of  the  book,  but  it  is  not  necessary  to  re- 
capitulate them. 

In  revising  the  book  for  a fourth  edition,  the  object 
has  been  to  make  the  fewest  and  smallest  changes 
consistent  with  giving  a true  idea  of  the  present  state 
of  our  knowledge.  In  many  cases  progress  has  been 
made  since  the  author  revised  the  book  for  the  third 
edition,  but  except  when  this  progress  has  rendered 
the  text  in  any  way  misleading,  it  has  been  thought 
best  to  make  no  change,  and  merely  to  add  references, 
either  in  footnotes  or  at  the  ends  of  the  chapters,  to 
books  or  papers  in  which  the  later  work  is  described. 
Material  alterations  or  additions,  and  footnotes  other 
than  simple  references,  are  enclosed  in  square  brackets. 
The  only  considerable  alterations  to  the  text  are  on 
pp.  200-202,  269-271,  and  275-279. 

I wish  to  express  my  warmest  thanks  to  Dr.  Leonard 
Doncaster,  F.R.S.,  for  his  great  kindness  in  revising 


vu 


611889 


PREFACE  TO  THE  FOURTH  EDITION 


the  fourth  edition  and  for  making  those  alterations 
and  valuable  additions  which  the  lapse  of  time  rendered 
necessary.  I cannot  put  into  adequate  words  my 
appreciation  of  this  proof  of  the  friendship  which 
existed  between  Dr.  Doncaster  and  my  husband,  j 
am  also  indebted  to  Professor  Bateson,  F.R.S.,  for  his 
valued  advice  on  Millardet’s  hybrids. 

B.  L. 


NOTE  TO  FIFTH  EDITION 

In  revising  the  book  for  a fifth  edition,  I have  made 
only  slight  alterations.  A few  sentences  and  footnotes, 
referring  to  work  published  since  1916,  have  been 
added ; as  in  the  fourth  edition,  these  are  enclosed 
in  square  brackets.  The  only  other  alteration  that 
requires  mention  is  the  omission,  in  several  places,  of 
the  words  ‘ recent,’  ‘ recently  published,’  or  others  of 
similar  import,  when  they  referred  to  events  or  ideas 
now  some  fifteen  or  more  years  old,  and  of  expressions 
of  doubt  about  facts  which  have  subsequently  been 
definitely  established.  With  the  exception  of  these 
slight  textual  alterations,  made  advisable  by  the  lapse 
of  time,  and  of  the  passages  enclosed  in  brackets,  the 
book  remains  substantially  as  it  was  left  by  the  author. 

L.  D. 


PREFACE  TO  THE  FIRST  EDITION 

The  idea  of  writing  this  little  book  occurred  to  me 
whilst  reading  Mr.  W.  C.  D.  Whetham’s  volume  on 
‘ The  Recent  Development  of  Physical  Science.’  I 
found  the  story  of  the  modern  progress  of  physics  so 
interesting  as  to  encourage  the  belief  that  a similar 
account  of  the  subjects  with  which  I was  myself  more 
particularly  familiar  might  prove  of  a like  interest  to 
other  people.  I did  not,  indeed,  suppose  for  a moment 
that  I could  vie  with  Mr.  Whetham  in  the  power  ol 
literary  expression  which  renders  his  book  so  eminently 
readable.  I rather  hoped  that  the  peculiar  interest 
and  importance  of  the  theme  might  outweigh  the 
present  author’s  deficiencies  in  this  respect. 

For  the  group  of  subjects  of  which  I intended  to  give 
a brief  account  Mr.  W.  Bateson  has  recently  proposed 
the  term  ‘ genetics,’  an  expression  which  sufficiently 
indicates  their  scope  to  the  initiated.  Since,  however, 
the  meaning  of  the  word  ‘ genetics  ’ is  not  yet  clearly 
understood  by  everybody,  it  seemed  better  to  adopt 
in  the  present  instance  a somewhat  more  descriptive 
title. 

b 

IX 


PREFACE  TO  THE  FIRST  EDITION 


The  rediscovery  of  Mendel’s  law  some  seven  years 
ago  led  to  a complete  change  in  our  attitude  towards 
the  problems  of  variation,  heredity,  and  evolution ; 
and  the  new  method  of  study  thus  introduced  has 
rendered  possible  a renewal  of  the  attack  upon  these 
problems  with  increased  vigour  and  with  remarkable 
results.  At  the  present  time  this  activity  may  be 
said  to  have  its  centre  in  the  school  of  genetic  research 
founded  at  Cambridge  by  the  independent  energy  of 
Mr.  Bateson.  So  far-reaching  are  the  results  already 
arrived  at  by  Mr.  Bateson  and  others,  both  in  their 
scientific  interest  and  in  their  probable  influence  upon 
human  affairs,  that  it  seemed  desirable  to  give  an 
immediate  account  of  these  and  of  kindred  fines  of 
recent  study,  even  though  the  rapid  progress  which  is 
a characteristic  manifestation  of  this  department  of 
science  must  render  any  such  attempt  a more  or  less 
transitory  one.  ^ 

Whilst  I was  still  engaged  upon  my  task,  the  first 
volume  of  Dr.  Lotsy’s  admirable  ' Vorlesungen  fiber 
Descendenztheorien  ’ made  its  appearance.  But  for 
the  fact  that  most  of  the  following  pages  had  then 
already  been  written,  I might  have  hesitated  to  pursue 
my  project,  since  a book  not  altogether  unlike  the 
present  might  be  produced  by  the  comparatively  simple 
process  of  making  a series  of  judicious  extracts  from 
Dr.  Lotsy’s  work.  The  latter  is,  however,  in  the 
German  language,  and  on  a considerable  scale,  so  that 
there  seemed  still  to  be  room  for  an  introduction  to 


PREFACE  TO  THE  FIRST  EDITION 


the  science  of  genetics  of  the  more  modest  dimensions 
which  I had  contemplated.  I should  wish,  however, 
particularly  to  recommend  Dr.  Lotsy’s  lectures  to  any 
reader  who  wishes  to  go  further  into  these  matters. 

I am  indebted  to  several  friends  for  assistance  during 
the  course  of  my  work.  Mr.  R.  P.  Gregory  kindly 
read  through  the  proof  of  the  chapter  on  cytology  ; and 
I wish  here  to  record  my  thanks  to  Mr.  J.  Stanley 
Gardiner,  to  Mr.  C.  T.  Regan,  to  Mr.  W.  S.  Perrin,  and 
to  Mr.  R.  C.  Punnett  for  information  on  special  points. 
To  the  last-named  I owe  the  photograph  which  appears 
as  Fig.  15.  I am  particularly  grateful  to  Mr.  R.  H. 
Biffen  and  to  Mr.  G.  Udny  Yule  for  access  to  work 
which  has  not  hitherto  appeared  in  print. 

Adequately  to  acknowledge  Mr.  Bateson’s  influence 
upon  these  pages  is  a more  difflcult  matter,  and  not 
the  less  so  because  I have  deliberately  refrained  as 
far  as  possible  from  consulting  him  whilst  the  book 
was  in  course  of  preparation,  in  order  that  it  might 
retain  if  possible  some  traces  of  individuality.  It  is 
therefore  clear  that  he  is  in  no  way  responsible  for  its 
deficiencies.  But,  apart  from  the  fact  that  I am 
conscious  of  having  quoted  his  ideas  at  more  points 
than  could  possibly  be  acknowledged  seriatim,  I owe 
to  Mr.  Bateson  both  my  first  introduction  to  the  science 
of  genetics,  and  a continual  fund  of  encouragement  in 
the  prosecution  of  studies  connected  with  it. 

I have  to  thank  Mr.  Francis  Darwin  for  kind  per- 
mission to  reproduce  a portrait  of  his  father ; Professor 

XI 


PREFACE  TO  THE  FIRST  EDITION 

de  Vries  for  the  present  of  an  excellent  portrait ; and 
Mr.  Francis  Galton  for  the  loan  of  a photograph  well 
known  from  earlier  reproductions.  The  portrait  of 
Mendel  is  reproduced  from  the  frontispiece  to  Mr. 
Bateson’s  ' Defence,'  by  the  permission  of  the  Syndics 
of  the  Cambridge  University  Press.  Messrs.  Macmillan 
and  Co.  have  kindly  allowed  the  reproduction  of  the 
diagram  which  occupies  p.  87,  and  of  the  table  and 
figure  on  pp.  89  and  90.  The  figures  facing  pp.  143 
and  150  are  from  de  Vries’  ‘ Mutationstheorie,’  pub- 
lished by  Messrs.  Veit. 

The  attempt  has  been  made  to  render  the  following 
pages  intelligible  to  the  general  reader,  as  well  as  to 
the  more  scientific  public,  to  which  they  are  primarily 
addressed.  A short  glossary  has  been  added,  which 
may  be  found  useful  by  those  who  have  no  previous 
acquaintance  with  biological  terms. 

Cambridge, 

October  23,  1906. 


THE  LIBRARY 
Of  THE 

UNIVERSITY  Of  ILLIftOil 


\To  face  p.  xiii. 


BIOGRAPHICAL  NOTE 


‘ We  live  in  deeds,  not  years  ; in  thoughts,  not  breaths  | 

In  feelings,  not  in  figures  on  a dial. 

We  should  count  time  by  heart-throbs.  He  most  lives 
Who  thinks  most,  feels  the  noblest,  acts  the  best.^ 

Philip  J.  Bailey. 

It  has  been  suggested  that  a few  details  of  the 
author’s  life  might  fittingly  preface  this  fourth 
edition,  which  he  was  not  fated  to  see  through  the 
press. 

Robert  Heath  Lock  was  the  third  son  of  the  Rev. 
John  Bascombe  Lock  and  Emily,  daughter  of  Edwin 
Baily  of  Cirencester.  He  was  born  on  January  19, 
1879,  at  Eton,  where  his  father  was  at  that  time  a 
master.  ‘ R.  H.,’  as  he  was  known  to  many  of  his 
friends,  was  educated  at  Mr.  Stone’s  school  at  Broad- 
stairs,  and  at  Charterhouse.  He  was  a quiet,  sensitive 
child  and  boy,  finding  from  an  early  age  great  pleasure 
in  books.  Still,  he  was  not  too  much  of  a bookworm  to 
revel  in  the  world  of  sport  and  games.  All  his  life  he 
enjoyed  outdoor  pastimes  with  the  keenness  and  zest 
that  he  threw  into  everjdhing  he  did.  At  Charter- 
house  he  was  head  monitor  of  Hodgsonites  and  got  his 
second  eleven  colours  for  football.  But  it  was  as  a 

xiii 


BIOGRAPHICAL  NOTE 


shot  that  he  scored  his  great  success.  He  fired  the 
winning  shot  for  the  Ashburton  Shield  won  by  Charter- 
house  at  Bisley  in  1893, 

Most  boys  find  considerable  difficulty  in  choosing 
a career,  and  pass  through  various  phases  when  soldier- 
ing, sailoring,  or  engine-driving  seem  an  ideal  existence, 
and  many  of  them  eventually  are  ‘ put  into  something  ’ 
by  parents  or  friends.  But  while  still  at  school 
R.  H.  Lock’s  bent  for  science  was  already  very  marked, 
and  there  was  no  question  of  any  other  career  for  him. 

R.  H.  Lock  came  up  to  Caius  as  an  Exhibitioner 
for  Natural  Science  in  1898,  and  entered  on  a very 
happy  period  of  college  life.  He  took  a First  Class 
% the  Part  I.  of  the  Natural  Science  Tripos  in  1900, 
thereby  becoming  a Scholar  of  the  College,  and  a 
First  Class  in  Part  II.  (Botany)  in  1902.  He  was 
awarded  a Frank  Smart  Studentship,  and  was  able  to 
gratify  the  ‘ wander-spirit  ’ which  was  strong  in  him 
by  going  to  Ceylon  for  research  work. 

He  worked  at  Peradeniya  under  Dr.  Willis,  Director 
of  the  Royal  Botanic  Gardens,  and  the  result  of  his 
experiments,  especially  in  the  cross-breeding  of  maize, 
won  him  in  1902  a Fellowship  at  Caius,  where  his  father 
was  also  a Fellow.  He  wandered  home  via  China 
Japan,  and  America,  enjoying  the  trip  in  his  own 
quiet,  observant  way.  The  next  four  years  were  spent 
at  Cambridge,  working  in  the  Botanical  Laboratory 
(where  he  was  appointed  Curator  of  the  Herbarium 
on  material  he  had  brought  back  from  Ceylon.  He  also 
wrote  at  this  time  the  book  ‘ Variation,  Heredity,  and 
Evolution,’  to  which  these  words  are  prefaced. 


XIV 


BIOGRAPHICAL  NOTE 


In  1908  he  returned  to  Ceylon  at  the  request  of 
Dr.  Willis,  as  Assistant  Director  of  the  Royal  Botanic 
Gardens.  Dr.  Willis  first  formed  the  idea  of  collecting 
at  Peradeniya  a staff  dealing  with  entomology,  plant- 
breeding, mycology,  etc.,  ready  to  experiment  for, 
and  give  advice  to,  the  planter  and  the  native  culti- 
vator— an  idea  which  has  been  successfully  imitated 
in  India. 

In  1910  R.  H.  Lock  returned  to  England  on  short 
leave,  when  he  married  Bella  Sidney  Woolf,  eldest 
daughter  of  the  late  Sidney  Woolf,  Q.C.  He  also 
took  his  degree  of  Sc.D.  He  returned  to  Ceylon  to 
take  up  residence  in  a charming  bungalow  that 
Government  had  built  for  him  in  the  Peradeniya 
Gardens — an  ideal  spot  to  live  in.  From  1908  to 
1912  his  work  was  very  strenuous.  For  a considerable 
period  he  was  Acting-Director  of  the  Gardens  in  the 
absence  of  the  Director  on  leave.  At  the  same  time 
he  conducted  extensive  experiments  in  rubber-tapping 
and  paddy-breeding. 

The  result  of  the  latter  research  work  was  a new  strain 
of  rice,  known  as  ‘ Lock’s  paddy.’  This  variety  is  now 
displacing  other  varieties  throughout  the  island.  He 
is  the  only  official  connected  with  the  Botanic  Gardens 
throughout  the  hundred  years  and  more  of  their 
existence  who  has  given  his  name  to  any  product. 

But  changes  were  taking  place  which  led  eventually 
to  R.  H.  Lock’s  resignation  of  his  post.  Government 
decided  to  create  a Department  of  Agriculture,  and 
to  place  the  Gardens  under  its  administration.  Dr. 
Willis  retired,  and  R.  H.  Lock  was  offered  the  post  of 


BIOGRAPHICAL  NOTE 


Superintendent  of  the  Gardens.  He  felt  that  this  was 
a post  out  of  his  "line  ’’  and  special  interests,  and  one 
distinctly  fitted  for  a systematic  botanist.  He  there- 
fore refused  it. 

He  returned  to  England  and  spent  a few  months  in 
writing  a book  on  ‘ Rubber  and  Rubber  Planting  ’ 
(Camb.  Univ.  Press) — the  result  of  his  extensive 
experiments  and  study  of  rubber  during  his  time  in 
Ceylon.  He  then  accepted  an  Inspectorship  under  the 
Board  of  Agriculture  and  Fisheries.  He  was  given 
charge  of  Birmingham  and  district,  Shropshire,  Stafford- 
shire, and  the  fruit-  and  hop-growing  counties  of 
Worcestershire,  Herefordshire,  and  Gloucestershire. 
Pie  threw  himself  into  his  work  with  his  usual  ardour. 
An  Inspector  under  the  Board  of  Agriculture  has  to 
travel  continuously  through  his  extensive  district, 
with  the  object  of  persuading  farmers  and  fruit  and 
hop  growers  to  check  the  spread  of  plant  diseases. 
R.  PI.  Lock  was  a keen  and  expert  motorist,  and  he 
travelled  thousands  of  miles  in  his  two-seater  car, 
enjoying  the  beauties  of  the  English  countryside  after 
his  sojourn  in  the  Tropics.  He  always  exerted  in  his 
work  a very  powerful  influence  upon  those  with  whom 
he  came  in  contact,  and  as  an  Inspector  this  was 
particularly  noticeable.  A friend  of  his — a well-known 
Professor  of  Agriculture — wrote : 

‘ In  the  summer  of  1913  I spent  a day  with  him 
visiting  Birmingham,  and  was  much  interested  in 
observing  the  quiet  and  forceful  manner  in  which  he 
got  his  ovm  way.’ 

As  he  had  now  little  time  or  opportunity  for  sport. 


BIOGRAPHICAL  NOTE 


he  followed  the  advice  of  his  favourite  Candide,  and 
cultivated  his  garden  in  his  leisure.  He  put  his 
characteristic  energy  and  thoroughness  into  this  hobby, 
and  obtained  correspondingly  good  results. 

Wlien  war  broke  out,  R.  H.  Lock  was  one  of  the 
first  to  feel  he  must  join  the  army.  He  had  shot  for 
his  College  when  he  was  a member  of  the  University 
Volunteers,  and  he  had  been  a member  of  the  Ceylon 
Mounted  Rifles.  But  Government  requested  him  to 
undertake  special  war  work.  He  was  appointed 
Chairman  of  a Vegetable  Drying  and  Fruit  Preserving 
Committee,  and  he  threw  himself  heart  and  soul  into 
the  immense  amount  of  work  that  arose.  Two 
factories  were  started  by  the  Committee  with  a view 
to  experimenting  on  the  drying  and  preserving  of 
fruit  and  vegetables,  thus  conserving  and  increasing 
the  national  food-supply,  and  providing  attractive 
and  nutritive  food  for  the  troops.  The  work  interested 
him  keenly,  for  he  united  in  a remarkable  degree 
scientific  and  business  abilities.  He  was  intensely 
practical  in  everything  he  did.  Naturally  the  lion’s 
share  of  work  and  responsibility  devolved  on  the 
chairman.  In  addition,  he  had  his  own  work  of 
Inspector  rendered  still  heavier  by  the  shrinkage  of 
the  staff  of  the  Board,  many  of  whom  were  with  the 
colours. 

In  February,  1915,  at  the  most  interesting  and 
crucial  stage  in  the  development  of  the  factories, 
R.  H.  Lock  was  struck  down  by  a severe  attack  of 
influenza,  but  he  refused  to  give  in  for  more  than  a few 
days.  He  made  an  incomplete  recovery,  and  in 

xvii 


BIOGRAPHICAL  NOTE 


March  was  again  attacked  by  the  same  illness  accom- 
panied by  tonsilitis.  Even  then  he  refused,  in  the 
face  of  the  doctor’s  serious  warnings,  to  give  in  or 
to  take  a holiday.  He  felt  that  things  were  shaping 
well  and  he  could  not  abandon  them  just  then.  With 
extraordinary  fortitude  he  struggled  through  the  illness 
and  carried  on  his  work  till  June,  when  certain  dis- 
quieting symptoms  of  breathlessness  showed  them- 
selves. Several  specialists  were  consulted,  but  none  of 
them  could  actually  point  to  the  real  cause  of  the 
trouble.  The  opinions  inclined  to  overstrain  of  nerves, 
and  a holiday  was  insisted  on.  In  the  middle  of  June 
he  went  to  Eastbourne.  He  enjoyed  the  change,  but 
the  alarming  symptoms  did  not  abate.  Suddenly, 
without  a moment’s  warning,  he  died  of  heart  failure 
on  June  26,  1915- 

Kis  devotion  to  duty  cost  him  his  life.  As  one  of 
his  chiefs  at  the  Board  wrote  : 

‘ He  gave  his  life  for  his  country  as  much  as  if  he 
had  fallen  on  the  fields  of  France.’ 

It  was  only  given  to  a few  to  realize  the  extraordinary 
depths  and  beauty  of  his  nature.  He  was  shy  and 
reserved  in  society,  and  yet  in  his  own  four  walls  he 
was  full  of  whimsical  humour.  He  could  not  suffer 
fools  gladly,  and  his  hatred  of  humbugs  both  great 
and  small  was  as  fiery  as  Huxley’s.  An  old  college 
friend,  writing  about  the  photograph  which  prefaces 
this  note,  said : 

‘ The  photograph  is  quite  the  best  of  R.  H.  that  I’ve 
ever  seen.  I can’t  tell  you  how  glad  I am  to  have  it. 
He  looks  as  though  he  were  imploring  me  not  to  be 

xviii 


BIOGRAPHICAL  NOTE 


merely  specious  in  my  argument — a thing  which  in 
the  old  days  often  happened  ! I am  so  glad  the 
photo  has  this  characteristic  expression.  I think, 
in  a very  charming  way,  it  reflects  the  outstanding 
feature  of  his  moral  and  mental  personality.  He 
liked  cleverness,  but  wouldn’t  put  up  with  it  for  a 
moment  if  it  were  superficial.  His  love  of  truth  was 
something  more  than  the  habit  of  a trained  scientist.’ 

He  would  never  go  out  of  his  way  by  a hair’s-breadth 
to  conciliate  or  pander  to  ‘ useful  ’ people  whom  he 
disliked  in  order  to  advance  his  interests.  He  was 
tittiid  socially,  but  he  was  absolutely  fearless  where 
the  rights  and  wrongs  of  life  were  concerned. 

He  would  not  ‘ strike  leagues  of  friendship  with 
cheap  persons,  where  no  friendship  can  be,’  but  he 
had  a few  friends  to  whom  he  stood  in  the  closest 
relation.  His  best  friend  wrote : ‘ He  was  more  to 
me  than  a brother.’ 

From  those  over  whom  he  was  set  in  authority  he 
won  the  most  loyal  and  devoted  service.  This  applied 
to  the  native  staff  at  Peradeniya  no  less  than  to  his 
Sub-Inspectors  of  the  Board  of  Agriculture.  He  was 
always  considerate,  just,  and  above  all  anxious  to  give 
every  man  credit  for  work  well  done.  His  staff  knew 
that  he  never  spared  himself,  and  they  had  a great 
respect  for  his  untiring  energy  and  his  thoroughness. 

The  following  beautiful  tribute  came  to  me  from  one 
of  his  Sub-Inspectors : 

‘ Although  it  is  only  a little  over  a year  since  I first 
knew  him,  yet  I feel  that  I have  lost  a valuable  friend. 
He  was  more  like  a friend  than  a supervisor,  for  he 


XIX 


BIOGRAPHICAL  NOTE 


was  always  most  considerate;  and  the  year  I spent 
under  his  supervision  I shall  always  look  back  upon 
as  one  of  the  happiest  years  of  my  life,  and  I am  sure 
the  influence  and  knowledge  gained  will  be  a valuable 
asset  to  me  in  the  future. 

‘ He  has  also  left  behind  him  many  friends  amongst 
the  market  gardeners,  etc.,  here,  for  on  all  hands  I 
hear  genuine  expressions  of  regret  at  his  untimely 
death.’ 

Pie  was  entirely  unselfish  in  daily  life,  and  tender 
and  humane  towards  all  living  things.  Although  he 
was  a good  shot  at  the  ranges,  during  all  the  years  in 
Ceylon  he  never  ‘ went  out  to  kill  something.’  Killing 
for  sport  had  no  attraction  for  him.  He  preferred 
golf,  and  tennis,  and  football — in  all  these  games 
he  was  proficient  above  the  average. 

In  his  thirty-six  years  he  had  achieved  much. 
Men  of  mark  looked  upon  him  as  one  of  the  leading 
younger  men  in  the  world  of  agricultural  science. 
The  Board  of  Agriculture  destined  him  for  the  impor- 
tant post  of  British  Representative  at  Rome  on  the 
Council  of  the  International  Agricultural  Institute. 

To  England  the  loss  of  such  sons  is  great ; he  gave  up 
his  whole  life  to  the  service  of  his  country — and  laid  it 
down  for  her.  No  man  ever  had  a stronger  sense  of 
duty.  To  those  who  knew  him  well  he  was  a source 
of  inspiration  and  strength.  He  never  preached  or 
posed.  He  was  just  himself,  incapable  of  affectation  or 
pretence. 

B.  L. 

June,  1916. 

XX 


RECENT  PROGRESS 
IN  THE  STUDY  OF  VARIATION, 
HEREDITY,  AND  EVOLUTION 


CHAPTER  I 

INTRODUCTION 

The  present  volume  deals  with  variation  and  inheii- 
tance  in  plants  and  animals,  especially  in  so  far  as 
those  subjects  bear  upon  the  problem  of  the  origin  of 
species.  By  inheritance  we  mean  those  methods  and 
processes  by  which  the  constitution  and  characteristics 
of  an  animal  or  plant  are  handed  on  to  its  offspring, 
this  transmission  of  characters  being,  of  course,  asso- 
ciated with  the  fact  that  the  offspring  is  developed  by 
the  processes  of  growth  out  of  a small  fragment  de- 
tached from  the  parent  organism.  The  term  ‘ varia- 
tion,’ on  the  other  hand,  includes  a number  of  different 
phenomena  which  will  be  described  at  greater  length 
as  the  work  proceeds  ; but,  broadly  speaking,  we  may 
say  that  the  study  of  variation  is  concerned  with  the 
circumstance  that  members  of  the  same  species  are 
not  aU  alikei  and  more  particularly  with  the  fact  that 

I 


2 


INTRODUCTION 


differences  are  to  be  found  between  different  members 
of  the  same  family.  Some  of  these  differences  arise 
comparatively  late  in  life,  and  may  be  the  result  of 
circumstances  or  of  education.  It  is  the  first  duty  of 
the  student  of  variation  to  distinguish  as  far  as  may 
be  possible  between  differences  of  this  kind  on  the  one 
hand,  and  those  differences  on  the  other  which  depend 
upon  the  fact  that  the  different  detached  fragments, 
\f  as  we  have  termed  them,  of  the  parent  organism — its 
germ-cells,  in  fact — show  greater  or  smaller  differences 
among  themselves. 

The  facts  of  variation  have  this  very  special  impor- 
tance, that  the  whole  theory  of  organic  evolution  is 
based  upon  them.  The  fact  that  members  of  the  same 
species  are  not  all  alike,  depending  upon  the  further 
fact  that  offspring  may  differ  from  their  parents,  makes 
it  possible  in  the  course  of  generations  for  progressive 
changes  to  take  place,  so  that  from  the  offspring  of 
different  members  of  the  same  species  different  new 
species  may  arise.  But  for  this  fact  of  variation  it 
would  have  been  quite  impossible  for  Darwin'^to  have 
( overthrown  the  former  crude  belief  in  a fecial  crea- 
^ tion  of  each  separate  species,  since  there  would  have 
been  no  material  for  his  great  factor — natural  selection 
— ^to  work  upon.  It  is  with  variation,  then,  and  with 
the  manner  in  which  characters  appear  in  the  succes- 
sive generations  of  living  things,  that  we  are  here 
concerned. 

Ever  since  the  publication  of  Darwin’s  ‘Origin  of 
Species  ’ in  1859,  these  subjects,  and  especially  the 
theoretical  aspects  of  them,  have  been  received  even 


THE  STUDY  OF  EVOLUTION 


3 


by  the  general  public  with  all  the  signs  of  a genuine 
enthusiasm ; and  nonej  moreovetj  can  be  more  fas- 
cinating to  the  professional  naturalist.  But  since  the 
time  of  Darwin  the  more  popular  accounts  have  dealt 
almost  exclusively  with  theoretical  considerations  and 
with  matters  of  opinion.  Highly  abstruse  contro- 
versies have  raged  freely  between  Neo-Lamarckians 
and  Neo-Darwinians^  and  these  have  found  a place 
in  the  pages  of  works  ostensibly  intended  for  the  in- 
struction of  all  and  sundry ; whilst  only  a bare  re- 
siduum of  actual  matters  of  fact  has  seen  the  light  of 
popular  publication.  If  the  truth  must  be  told^  the 
experimental  method  was  given  up  for  a long  time  by 
the  majority  of  specialists  themselves  in  favour  of  the 
controversial,  and,  indeed,  this  tendency  has  by  no 
means  yet  died  out  from  among  the  habits  of  some 
professed  evolutionists.  On  the  other  hand,  during 
the  last  fifteen  to  twenty  years,  a few  scattered  workers 
have  diligently  applied  themselves  to  a study  of  the 
facts  of  variation  and  inheritance,  with  results  which 
already  more  than  justify  the  anticipation  in  which 
their  work  was  begun — namely,  that  by  such  methods 
alone  can  any  real  progress  in  our  knowledge  of  the 
processes  of  evolution  be  brought  about. 

The  science  of  organic  evolution  is  by  no  means  the 
simple  and  isolated  study  it  might  be  supposed  to  be 
from  a perusal  of  some  of  the  more  popular  accounts. 
Its  footing  rests  immediately  upon  the  widest  founda- 
tions which  zoology,  botany,  and  physiology  can  afford  ; 
and  these  in  their  turn  are  ultimately  based  upon  the 
results  of  chemical  and  physical  science.  But  some 

I — 3 


4 


INTRODUCTION 


of  the  most  fundamental  parts  of  physical  science,  as 
I think  we  may  fairly  call  the  branches  of  electricity 
and  molecular  physics,  seem  at  present  to  be  under- 
going modifications  which  bid  fair  to  bring  about  a 
complete  revolution  in  current  ideas  upon  these  sub- 
jects. It  is  highly  probable  that  these  results  will 
ultimately  lead  to  a considerable  modification  in  pre- 
vailing notions  about  living  things ; but  the  new 
developments  have  yet  to  reach  biology  through  the 
channels  of  organic  chemistry^  physiology^  cytology, 
and  the  like,  and  at  present  we  do  not  know  what  the 
result  of  this  influx  is  likely  to  be.  These  considera- 
tions need  not,  however,  detain  us,  for  the  new  know- 
ledge of  variation  and  inheritance,  of  which  it  is  pro- 
posed to  give  some  account,  is  largely  concerned  with 
the  grosser  characters  of  organisms,  so  that  ultraminute 
structures  may  be  left  alone  for  the  present  until  the 
stream  of  physical  knowledge  stirs  them  into  greater 
prominence.  So  much  is  this  the  case  with  the  study 
of  variation  and  inheritance  by  experimental  methods 
at  the  present  day,  that  this  science  is  treated  by  some 
with  a fine  contempt,  because  its  tools  are  those  of  the 
breeder  and  gardener,  and  because  the  assistance  of 
the  compound  microscope  may  often  be  laid  aside  for 
days  together.  Yet  this  applies  only  to  one  aspect  of 
the  subject,  and  the  microscopic  study  of  the  embryonic 
rudiments  of  organisms,  going  hand  in  hand  with  the 
experimental  observation  of  adult  structures,  is  rapidly 
leading  to  a clearer  understanding  of  the  processes  of 
heredity. 

The  problem  which  those  who  are  engaged  in  this 


THE  PROBLEM  OF  SPECIES 


5 


kind  of  work  have  set  themselves  for  solution  is  that 
of  the  nature  and  method  of  origin  of  the  existing 
differences  between  certain  groups  of  organic  beings 
— namely,  species.  • Basing  their  studies  on  the  doc- 
trine that  the  present  species  have  arisen  through  the 
modification  of  pre-existing  species,  they  endeavour 
to  observe  how  modifications  of  existing  species  do 
actually  arise  in  Nature,  as  well  as  under  domestica- 
tion ; and  they  watch  the  hereditary  transmission  of 
the  modified  forms  when  like  is  bred  with  like,  and 
when  different  types  are  crossed  together.-  For  the 
theory  of  uniformity,  now  universally  accepted,  teaches 
us  that  the  organisms  with  which  we  are  now  familiar 
owe  their  present  characteristics  to  the  accumulation 
of  a series  of  changes  similar  to  those  which  are  still 
in  progress.  It  has,  thereforej  appeared  likely  to  a 
few  that  a further  understanding  of  the  processes  of 
evolution  might  best  be  obtained  by  a closer  study< 
firstly,  of  variationj  or  the  ways  in  which  offspring 
differ  from  their  parents  ; and,  secondly,  of  inheritance, 
or  the  ways  in  which  the  resemblances  between  parents 
and  their  offspring  are  perpetuated  from  one  genera- 
tion to  another. 

It  may  be  well  to  point  out  at  once  that  the  further 
study  of  the  method  of  origin  of  new  species,  admitting, 
as  it  does,  that  this  process  is  not  yet  by  any  means 
fully  understood,  does  not  for  this  reason  imply  that 
the  theory  of  organic  evolution  itself  is  open  to  criti- 
cism. The  evidence  that  new  species  arise  by  the 
modification  of  pre-existing  species  is  quite  indepen- 
dent of  the  evidence  that  this  process  invariably  occurs 


6 


INTRODUCTION 


by  the  action  of  natural  selection  upon  minute  differ- 
ences, in  the  manner  which  Darwin  described,  and 
which  has  been  claimed  by  others  as  the  sole  means 
by  which  the  origin  of  new  forms  takes  place.*  The 
evidences  of  evolution  are  much  more  numerous  and 
more  weighty  than  the  evidences  of  the  survival,  of 
the  fittest.  ' The  theory  of  evolution,  as  opposed  to 
the  creation  hypothesis,  is  supported  by  innumerable 
facts  of  classification,  of  morphology,  and  of  embryo- 
logy, by  the  geographical  distribution  of  animals  and 
plants,  and  by  their  succession  in  the  geological  strata, 
as  weU  as  by  direct  observation  of  the  actual  occurrence 
of  changes  in  the  case  of  domestic  productions  as  well 
as  under  Nature,  and  many  of  these  facts  have  no 
direct  bearing  upon  the  theory  of  natural  selection. 

Before  discussing  the  problem  of  the  origin  of  species, 
it  is  necessary  to  arrive  at  some  idea  as  to  what  the 
term  ‘ species  ’ means.  And  this  is  not  altogether  an 
easy  matter,  since  a precise  definition  has  not  been, 
and  cannot  be,  agreed  upon.  The  idea  of  species  is, 
indeed,  of  great  antiquity  and  very  gradual  growth. 
Primitive  men  doubtless  recognised  certain  plants  or 
animals  as  being  like  one  another,  and  different  from 
others,  and  they  gradually  came  to  distinguish  such 
forms  by  giving  a different  name  to  each.  The  names 
first  used  must  have  applied  as  a rule  to  genera  rather 
than  to  species.  Thus,  such  common  names  of  plants 
as  rose,  bramble,  vetch,  nettle,  dock,  crowfoot,  are 
names  of  genera— groups  of  greater  extent  than  species^ 
and  often  more  easily  defined  than  the  latter.  Later 


LINNiEAN  SPECIES 


7 


on  civilized  men  paid  closer  attention  to  tne' different 
kinds  of  plants',  and  the  old  herbalists  discovered  and 
described  a number  of  different  sorts  of  roses,  of  butter- 
cups, and  of  other  plants,  and  distinguished  each  by  a 
descriptive  sentence. 

As  more  and  more  species  came  to  be  described,  this 
method  of  designation  became  very  cumbersome,  until 
Linnaeus,  about  the  middle  of  the  eighteenth  century, 
adopted  the  idea  of  a binomial  nomenclature  (originally 
suggested  by  Bachmann),  in  which  every  species  of 
each  known  genus  received  a separate  name  of  its  own 
to  distinguish  it,  so  that  the  different  kinds  of  butter- 
cups were  now  known  as  Ranunculus  acris,  R.  bulbosus, 
R.  sceleratus,  and  so  on. 

Linnaeus  himself  appears  to  have  had  a very  definite 
idea  of  what  constituted  a species,  and  in  accordance 
with  the  view  then  current,' he  defined  a species  as 
being  a group  of  organisms  which  owed  its  origin  to  a 
separate  act  of  creation. ' From  the  nature  of  the  case 
this  definition  could  be  of  little  use  in  practice.  Prac- 
tically, then,  species  were  defined  as  groups  of  animals 
or  plants,  the  members  of  which  resembled  one  another 
in  definite  morphological  characteristics — that  is  to 
say,  in  constant  features  of  form  and  structure.  This 
definition  has  survived  the  downfall  of  the  dogma  of 
the  constancy  of  species,  and  at  the  present  day  species 
as  defined  by  Linnaeus  are  found  to  be  groups  of  much 
merit  both  for  naturalness  and  for  convenience — at 
any  rate  in  the  case  of  plants.  The  fact  that  inter- 
mediate forms  and  minor  groups  do  sometimes  and  to 
some  extent  bridge  over  the  gap  which  separates  a 


8 


INTRODUCTION 


pair  of  species  thus  defined  seems  to  have  caused  dis- 
quiet in  the  mind  of  Linnaeus  himself,  and  he  recom- 
mended his  disciples  to  have  no  dealings  with  these 
inferior  varieties,  as  being  beneath  the  dignity  of  4 
botanist  to  notice.  Of  late  years  these  minor  species 
have  excited  much  attention,  and  it  is  to  a study  of  ^ 
this  kind  of  species  in  particular  that  the  mutation 
theory  of  de  Vries  owes  its  origin,  as  will  be  told  in  a 
later  chapter. 

Such  minor  groups,  occurring  within  the  limits  of  a 
single  Linnsean  species,  and  subdividing  it  into  smaller 
collections  of  individuals,  were  made  the  object  of 
special  study  in  the  case  of  plants  by  the  French 
botanist,  Jordan  ; and  for  this  reason  they  are  some- 
\ times  referred  to  as  Jordan’s  species.  Jordan,  for 
example — though  the  example  is  indeed  an  extreme 
one — described  more  than  two  hundred  different  types, 
all  of  which  would  formerly  have  been  included  in 
the  single  Linnasan  species,  Draha  verna.  To  take  a 
more  familiar  instance.  We  find  in  the  ‘ British 
Flora  ’ of  Bentham  and  Hooker  the  primrose,  the 
cowslip,  and  the  true  oxlip,  all  described  as  varieties 
of  one  and  the  same  species ; yet  these  three  kinds  of 
plants  are  now  almost  universally  recognised  to  be 
as  good  species  as  any  in  nature.*  ' In  a similar  way, 
on  closer  investigation,  it  has  been  found  necessary  to 
split  up  a considerable  number  of  Linnaean  species, 
and  to  subdivide  each  into  several  species  of  smaller 
range. 

* A contrary  opinion  is,  however,  expressed  in  the  Journal 
of  Botany  for  July,  1906. 


JORDAN’S  SPECIES 


9 


It  has  already  been  pointed  out  that  Linnseus  him- 
self distinctly  deprecated  this  process  of  splitting. 
‘ Varietates  levissimas  non  curat  botanicus,’  said 
Linnaeus.  Jordan,  however,  applied  the  method  of 
experiment  to  many  of  the  species  of  his  own  defini- 
tion, and  having  transplanted  them  from  a variety 
of  localities  to  the  uniform  soil  of  a garden,  found  that 
they  preserved  their  distinctive  characters  and  came 
perfectly  true  to  seed. 

It  appears  then  that  Jordan’s  species  are  just  such 
true  and  constant  groups  as  those  of  Linnaeus.  They 
are  separated  from  one  another  by  definite  features  of 
form  and  structure,  only  these  differences  are  not  so 
wide  as  those  which  separate  Linnaean  species.  The 
latter  are,  indeed,  to  be  looked  upon  as  more  or  less 
artificial  groups  or  aggregates  of  these  physiological 
species,  as  Jordan’s  species  have  also  been  called.  The 
problem  of  the  origin  of  the  smaller  groups  is  clearly 
to  be  placed  before  that  of  the  origin  of  the  larger 
species. 

It  is  true  that  in  the  case  of  certain  groups  of 
animals  and  plants  there  would  appear  to  be  no  possi- 
bility of  drawing  hard  and  fast  lines  between  the 
species,  which  thus  seem  to  shade  gradually  one  into 
the  other.  There  is,  however,  a great  difference 
between  the  admission  that  certain  nearly  related 
species  are  difficult  or  impossible  to  separate  definitely, 
and  the  statement  that  there  is  no  true  distinction 
between  them,  and  the  latter  statement  is  one  which 
few  are  bold  enough  to  make.  The  case  stands  thus. 
We  know  that  great  numbers  of  large  groups  (classes 


10 


INTRODUCTION 


and  families)  of  animals  and  plants  exist,  in  which 
the  most  nearly  related  species  are  quite  definitely 
distinct  from  one  another.  In  other  classes  systema- 
tists  have  so  far  found  great  difficulty  in  framing 
definitions  of  specific  groups.  We  shall  see  later  on, 
though  at  first  sight  it  may  appear  almost  paradoxical, 
that  it  is  quite  possible  for  groups  to  be  perfectly 
distinct,  although  individual  members  of  them  may 
have  deviated  so  far,  each  from  its  proper  type,  as  to 
render  impossible  the  task  of  deciding  from  their 
appearance  which  group  any  of  these  individuals 
belong  to. 

Let  us  next  consider  a particular  example  of  a class 
of  animals  in  which  the  discrimination  of  species  is 
difficult  or  impossible.  This  is  said  to  be  the  fact 
with  the  majority  of  sessile  animals — such  animals 
as  resemble  plants  in  their  stationary  habit,  and  in 
no  case  are  the  problems  of  species  separation  more 
difficult  than  in  the  class  of  the  stony  corals.  Now, 
attempts  to  determine  the  species  of  corals  have  so 
far  been  made  almost  entirely  from  a study  of  what 
may  be  called  vegetative  characters — ^usually  from 
details  of  the  shape  and  structure  of  the  stony  skeleton 
of  the  animals.  How  far  these  features  may  be  affected 
by  external  circumstances  has  not  been  determined, 
but  it  must  be  noted  that  the  so-called  skeleton  is 
entirely  external  to  the  living  organism.  Now  we 
know  that  in  the  case  of  many  of  the  higher  plants 
vegetative  characters  are  extremely  liable  to  become 
modified  owing  to  the  action  of  the  environment. 
Differences  of  moisture,  light,  soil,  climate,  and  alti- 


DISCONTINUITY  OF  SPECIES 


II 


tude,  are  aU  capable  of  changing  the  general  appear- 
ance of  a plant  so  as  to  render  it  scarcely  recognisable. 
Fortunately,  in  the  case  of  the  higher  plants,  the 
floral  organs,  which  are  the  ones  chiefly  made  use  of 
for  purposes  of  specific  discrimination,  are  very  little 
liable  to  modification  by  external  conditions ; but 
in  the  corals  a similarly  stable  set  of  organs  does 
not  appear  to  have  been  discovered.  It  seems,  there- 
fore, hardly  fair  to  regard  the  example  of  the  corals 
as  affording  an  established  exception  to  what  we 
must  look  upon  as  the  general  rule — namely,  that 
species  are  on  the  whole  definite  and  discontinuous 
groups. 

As  a rule,  then,  the  species  riddle  presents  itself 
definitely  as  the  problem  of  the  existence  of  a series 
of  discontinuous  groups  of  creatures,  sharply  marked 
off  the  one  from  the  other,  and  often,  too,  existing 
among  surroundings  which  afford  no  corresponding 
discontinuity,  though  each  is  well  enough  fitted  for 
the  life  which  it  has  to  lead. 

The  problem  which  we  have  to  face  has  been 
enunciated  by  Bateson  in  the  form  of  the  two  following 
propositions ; 

‘ I.  The  forms  of  living  things  are  various,  and  on 
the  whole  are  discontinuous  or  specific. 

‘ 2.  The  specific  forms  on  the  whole  fit  the  places  ^ 
they  have  to  live  in. 

‘ How,’  he  continues,  ‘ have  these  discontinuous 
forms  been  brought  into  existence,  and  how  is  it  they 
are  thus  adapted  ? This  is  the  question  the  naturalist 
is  to  answer.  To  answer  it  completely  he  must  find 


12 


INTRODUCTION 


(i)  the  modes  and  {2)  the  causes  by  which  these  things 
have  come  to  pass.’ 

The  differences  between  existing  species  are  open 
to  study  in  more  than  one  way.  By  way  of  limiting 
the  discussion  for  the  present,  we  shall  consider  the 
case  of  plant  species  only  ; but  the  methods  of  study 
which  are  applicable  to  animal  species  are  of  quite 
similar  kinds. 

Four  methods  at  least  are  available.  Firstly,  that 
of  comparison  ; secondly,  the  method  of  statistical 
examination  ; thirdly,  the  method  of  cultural  experi- 
ment ; and  lastly,  that  of  cross-breeding. 

The  method  of  comparison  is  the  one  to  which  the 
ordinary  worker  in  descriptive  botany  is  almost  of 
necessity  confined.  In  this  way  plants  which  closely 
resemble  one  another  are  grouped  together  as  belong- 
ing to  the  same  species,  and  separated  from  others, 
the  appearance  of  which  is  different.  By  appearance 
is  not  meant  simply  the  general  habit  of  the  plants  ; 
all  morphological  features  whatever  may  be  used  for 
purposes  of  comparison,  and  the  most  minute  are 
often  of  the  greatest  importance.  But  the  systema- 
tist  who  works  only  in  this  way  knows  nothing  of  the 
real  relationships  between  the  plants  with  which  he 
is  dealing. 

■^Tien  a sufficient  number  of  specimens  is  available, 
the  methods  of  statistics  can  be  applied.  These 
involve  the  making  of  a series  of  accurate  measure- 
ments or  countings  of  the  parts  upon  which  depend 
the  supposed  differences  or  resemblances  of  the  plants 


METHODS  OF  STUDY 


13 


under  consideration.  The  resulting  numbers  are  then 
ranged  in  order  so  that  a precise  view  of  the  numerical 
characters  of  a large  number  of  specimens  can  be 
readily  obtained.  By  the  use  of  such  methods  valu- 
able information  is  often  to  be  arrived  at.  But  the 
same  limitation  affects  them  as  in  the  preceding  case. 

So  that  the  only  way  in  which  we  can  come  to  a 
definite  decision  as  to  whether  a given  putative 
species  does  or  does  not  represent  a definite  and  con- 
stant type  is  by  resorting  to  our  third  method,  that 
of  sowing  its  seeds  and  actually  rearing  its  progeny. 
And  this  is  not  so  simple  a matter  as  might  appear 
at  first  sight,  for  a great  many  precautions  have  to 
be  taken.  Thus  we  must  separately  sow  the  seeds  of 
many  different  individuals  of  the  species  which  we 
are  examining,  so  as  not  to  base  our  conclusions  upon 
a few  experiments  only.  But  in  many  cases,  even 
when  this  has  been  done,  we  should  only  know  one  of 
the  parents  of  our  seedlings — that  is  to  say,  in  cases 
where  the  pollen  for  fertilization  may  possibly  have 
been  conveyed  by  natural  agencies  from  a different 
plant.  In  such  a case  we  must  either  ensure  self- 
polhnation  by  isolating  our  plants,  or  we  must  arti- 
ficially provide  pollen  from  a separate  known  parent. 
If  under  these  circumstances  a particular  group  of 
plants  preserves  the  characteristic  differences  which 
distinguish  it  from  another  group  which  has  also  been 
grown  for  a number  of  generations  under  the  same 
conditions,  we  have  at  last  reasonable  grounds  upon 
which  to  base  the  opinion  that  we  are  dealing  with 
two  distinct  physiological  species,  even  though  the 


14 


INTRODUCTION 


visible  differences  between  them  may  seem  very  small 
to  an  untrained  eye. 

Lastly,  evidence  of  a confirmatory  nature  may  be 
obtained  by  observing  the  results  of  cross-fertilization 
between  a pair  of  closely  allied  species.  Much,  too, 
may  be  made  out  from  the  failure  of  such  experimentsj 
since  the  refusal  of  two  plants  to  breed  together  is 
generally  regarded  as  clear  evidence  of  their  specific 
distinctness.  But  for  this  reason  the  method  of  cross- 
breeding is  more  particularly  adapted  for  the  examina- 
tion of  forms  somewhat  nearly  related  to  one  another 
— for  example,  different  members  of  the  same  species. 

As  the  result  of  the  methods  presently  to  be 
described,  the  fact  has  been  established  that  two 
entirely  distinct  sorts  of  divergencies  may  appear 
among  members  of  a single  family.  Variations,  that 
is  to  say,  may  be  of  at  least  ,two  different  kinds.  In 
the  first  place  we  have  those  slight  differences  which 
invariably  distinguish  all  the  members  of  every  family 
— individual  variations  which  affect  every  part  and 
every  character.  Such  differences  are  known  as 
fluctuating,  normal,  or  continuous  variations.  As  an 
example  we  may  cite  the  variations  in  size  or  stature 
shown  by  the  various  members  of  any  purely-bred  race. 
When  a large  number  of  individuals  are  compared  in 
respect  of  a character  of  this  kind,  they  are  found  to 
fall  into  a continuous  series  ranging  from  a certain 
extreme  of  shortness  on  the  one  hand  to  an  extreme 
of  tallness  on  the  other.  Individuals  of  a medium 
height  , however,  are  usually  more  numerous  than  either 


VARIATION 


X5 

of  the  extreme  forms.  Some  further  account  of  the 
study  of  continuous  variations  will  be  given  in 
Chapter  IV. 

A second  kind  of  variation  is  variously  known  as 
abnormal,  definite,  and  discontinuous  variation,  and 
includes  what  are  known  as  sports  and  mutations. 
Such  variations,  as  is  indicated  by  the  terms  applied 
to  them,;  involve  definite  differences  usually  of  con- 
siderable amplitude.  A good  example  of  a discon- 
tinuous variation  would  be  afforded  by  the  appear- 
ance of  a child  having  six  fingers  in  a family  in  which 
this  abnormality  was  not  previously  known  to  occur. 
We  shall  pursue  the  discussion  of  discontinuous  varia- 
tion and  of  the  methods  of  perpetuation  of  the  types 
which  thus  arise  in  Chapters  V.  and  VII. 

A short  account  of  the  historical  development  of  the 
theory  of  organic  evolution  is  given  in  Chapter  II.  As 
alternatives  to  the  theory  of  the  special  creation  of 
each  individual  species,  a number  of  more  or  less  crude 
speculations  were  indulged  in  by  the  philosophers  of 
ancient  Greece.  Passing  to  more  recent  times,  three 
distinct  accounts  of  the  method  of  origin  of  specific 
differences  have  been  proposed  almost  within  the  last 
century,  and  each  of  these  theories  still  finds  a number 
of  supporters. 

I.  The  view  of  Lamarck,  published  first  in  i8oi,  and 
in  an  enlarged  form  in  1809,  was  briefly  as  follows  5 
Noticing  that  the  organs  of  men  and  other  animals  are 
increased  and  strengthened  by  use,  and  particularly 
by  conscious  use,  Lamarck  assumed  that  this  effect 


i6 


INTRODUCTION 


could  be  passed  on  by  inheritance  from  parent  to  off- 
springs and  so  accumulated  from  generation  to  genera- 
tion. In  the  case  of  animals  Lamarck  conceived  the 
prodiiction  of  a new  specific  form  to  take  place  in  the 
following  way  : Owing  to  some  change  of  external  con- 
ditionss  the  desire  to  perform  some  new  kind  of  action 
was  set  up  in  the  parent  species,  and  by  the  hereditary 
effect  of  the  striving  occasioned  by  this  desire  a modi- 
fication of  the  organs  affected  into  forms  better  fitted 
to  carry  out  the  new  function  was  gradually  achieved. 

Thus  Lamarck  supposed  that  snakes  were  evolved 
from  a pre-existing  type  of  animal  which  was  of  a much 
less  attenuated  shape,  and  which  possessed  two  pairs 
of  limbs  like  any  other  vertebrates.  And  he  supposed 
this  evolution  to  have  taken  place  owing  to  the  con- 
stant striving  of  these  animals  to  pass  through  narrow 
crevices ; the  effect  of  such  striving  being  inherited,  and 
so  accumulated  from  one  generation  to  another. 

In  the  case  of  plants,  in  which  conscious  effort  is 
precluded,  a similar  result  was  supposed  to  have  been 
attained  by  an  hereditary  accumulation  of  the  effects 
of  the  environment. 

2.  The  explanation  of  Darwin,  or  at  least  the  Neo- 
Darwinian  form  of  it,  as  interpreted  by  Wallace, 
Weismann,  and  others,  and  as  opposed  to  and  exclud- 
ing the  view  of  Lamarck,  was  as  follows  : Two  separate 
factors  are  primarily  concerned  : (i)  the  fact  of  fluc- 
tuating variation — ^the  fact  that  no  two  members  of 
the  same  family  ever  resemble  one  another  exactly  ; 
and  (2)  the  occurrence  of  a struggle  for  existence 
between  organisms  owing  to  the  geometric  rate  of 


THEORIES  OF  THE  ORIGIN  OF  SPECIES  17 


increase  of  living  things.  From  these  two  facts  it 
follows  that  when  a change  of  environment  takes  - 
place,  certain  members  of  an  existing  species  will  be 
somewhat  better  adapted  than  others  to  withstand 
the  new  conditions,  and  the  former  will  tend  to  survive 
to  the  exclusion  of  the  latter.  It  is  assumed  that 
during  a long  series  of  generations  this  process  will 
cause  a steady  change  in  the  character  of  the  species  in 
the  direction  of  better  adaptation  to  the  new  conditions. 

Thus  we  might  suppose  that  among  the  ancestors  of 
the  snakes  those  which  happened  to  possess  the  longest 
and  thinnest  bodies  and  the  smallest  limbs  had  the 
advantage  over  their  fellows  that  they  were  able  to 
crawl  through  narrower  holes,  and  that  for  this  reason 
a greater  number  of  them  survived  to  produce  off- 
spring. Here  we  have  a better  basis  for  reasoning 
than  the  supporters  of  Lamarck’s  doctrine,  because 
we  actually  know  that  longer  parents^  in  whom  this 
quality  was  apparently  not  the  result  of  taking  thought  j 
do  tend  to  produce  on  the  average  longer  offspring. 

3.  The  view  of  the  mutationists,  already  fore- 
shadowed by  Aristotle,  and  in  recent  years  especially 
associated  with  the  names  of  Bateson  and  de  Vries, 
expresses  the  conclusion  that  the  evolution  of  new 
species  has  taken  place  principally  by  the  help  of 
variations  of  the  discontinuous  kind.  By  this  process 
there  can  arise  at  a single  step  new  forms  which  have 
already  the  complete  and  definite  character  usually 
associated  with  a species  specially  adapted  to  particular 
conditions.  Of  these  new  forms,  those  which  happen 
to  be  fitted  for  their  surroundings  as  well  as  or  better 


2 


i8 


INTRODUCTION 


than  their  predecessors  will  survive,  whilst  those  which 
are  worse  will  be  destroyed  by  the  action  of  natural 
selection. 

Thus  it  would  be  an  appropriate  use  of  this  concep- 
tion to  seek  in  a mutation  the  explanation  of  the  final 
loss  of  the  much  reduced  limbs  presumably  exhibited 
by  comparatively  I'ecent  ancestors  of  the  family  of 
snakes.  This  final  loss  is  especially  difficult  to  under- 
stand on  the  Darwinian  theory.  Moreover,  changes  of 
a closely  similar  nature  are  not  hypothetical,  but  have 
actually  been  observed  to  take  place.  At  the  same 
time  it  must  not  be  supposed  that  mutations  are  con- 
fined to  the  loss  of  pre-existing  organs ; indeed,  the 
origin  of  a totally  new  organ  is  quite  inexplicable  on 
either  of  the  two  preceding  theories.  The  very  first 
inception  of  such  an  organ  must*  it  would  seem,  of 
necessity  be  sudden. 

After  giving  some  account  of  the  earlier  theories  of 
evolution,  we  shall  next  proceed  to  treat  of  those  sub- 
jects with  which  we  are  more  properly  concerned — 
that  is  to  say,  the  recent  experimental  observations 
on  variation  and  natural  inheritancej  together  with 
their  bearing  on  the  theories  of  evolution.  And  in  the 
first  place  we  shall  describe  some  recent  studies  which 
are  not  strictly  experimental,  but  which  nevertheless 
deal  to  some  extent  with  actual  facts — namely,  the 
statistical  study  of  variations,  particularly  of  con- 
tinuous variations.  This  subject  has  been  dignified 
by  a special  name,  and  is  now  described  as  the  science 
of  biometry. 


MENDEL’S  METHOD 


19 


Of  even  greater  interest,  however,  are  the  more 
strictly  experimental  researches  which  have  been 
published  during  the  present  century.  In  the  first 
place,  we  have  the  observations  of  de  Vries,  who  has 
introduced  a new  method  of  study — that  of  cultivating 
great  numbers  of  seedling  plants  with  the  object  of 
discovering  definite  new  forms  or  mutations  among 
their  number.  Lastly,  and  in  its  results  much 
the  most  important  of  all,  we  have  the  method  of 
Mendel,  published  half  a century  ago,  but  only  re- 
cently brought  into  prominence  owing  to  its  redis- 
covery and  confirmation  by  three  independent  workers 
— Correns,  Tschermak,  and  de  Vries.  This  method 
consists  in  the  cross  breeding  of  strains  of  plants  or 
animals  which  differ  in  definite  characters,  and  in  the 
statistical  examination  of  the  proportions  in  which 
these  characters  appear  among  the  offspring  obtained 
from  the  crosses. 

Further  experiments  on  the  lines  which  Mendel  in- 
dicated bid  fair  to  revolutionize  within  a few  years  the 
arts  of  the  breeders  of  plants  and  animals.  This  is 
due  to  the  fact  that  such  experiments  are  leading  to 
the  introduction  into  these  pursuits  of  a degree  of 
scientific  exactness  which  was  previously  altogether 
unforeseen.  The  change  in  our  ideas  regarding  the 
method  of  hereditary  transmission  of  characters,  which 
has  resulted  from  these  experiments,  has  been  aptly 
compared  with  the  change  brought  about  in  men’s 
understanding  of  the  science  of  chemistry  by 
Dalton’s  conception  of  the  atom.  For  the  rest  the 
new  experiments  tend  on  the  whole  to  confirm  the 

3 — 3 


20 


INTRODUCTION 


experience  of  practical  breeders ; only  the  elucida- 
tion of  one  simple  rule  of  inheritance  has  brought  into 
order  a host  of  phenomena,  which  were  previously  quite 
incapable  of  a coherent  explanation. 

The  experimental  results  with  which  it  is  the  pro- 
vince of  this  book  to  deal  are,  then,  firstly  those  of 
biometry*  or  the  statistical  study  of  variations,  and 
particularly  of  continuous  variations  ; secondly,  the 
results  of  direct  observations  bearing  upon  the  origin 
of  species  by  the  discontinuous  method  ; and  thirdly, 
the  results  of  experimental  observations  on  heredity 
by  the  methods  of  scientific  breeding.  By  these 
methods  results  of  the  utmost  moment  to  mankind 
have  been,  and  are  being,  arrived  at,  quite  apart  from 
their  interest  as  bearing  upon  the  problems  of  evolu- 
tion. From  a biologist’s  point  of  view,  however,  the 
latter  is,  of  course,  paramount.  And  so  it  has  been 
thought  fitting  to  begin  with  a brief  discussion  of  the 
problems  of  evolution,  and  of  the  various  solutions  of 
them  which  have  been  from  time  to  time  suggested. 

In  a later  chapter  some  of  the  more  prominent  recent 
results  of  the  kindred  science  of  C3d:ology — the  micro- 
scopic study  of  the  minute  constituent  parts  of  organ- 
isms— ^will  be  briefly  described,  on  account  of  the  very 
close  connection  which  recent  progress  in  this  subject 
bears  to  the  experimental  study  of  the  inheritance  of 
the  grosser  characters. 

Finally,  we  have  essayed  a brief  account  of  the 
science  to  which  the  name  of  ‘ Eugenics  ' has  recently 
been  applied  by  Sir  Francis  Galton.  In  the  chapter 
dealing  with  Eugenics  an  attempt  is  made  to  show 


EUGENICS 


21 


how  far  our  recently  acquired  knowledge  of  variation 
and  heredity  can  be  applied  to  mankind,  and  to  ' the 
study  of  agencies  under  social  control  that  may  im- 
prove or  impair  the  racial  qualities  of  future  genera- 
tions/ 

BIBLIOGRAPHY. 

The  two  following  works  will  be  found  specially  useful  on 
account  of  the  extensive  bibliographies  which  they  contain  : 

Morgan,  T.  H.  : Experimental  Zoology,  1907. 

Thomson,  J.  A.  : Heredity,  revised  edition,  1919. 

These  may  be  consulted  for  further  references  beyond  those 
given  at  the  end  of  each  of  the  following  chapters. 

As  an  introduction  to  the  whole  subject  of  Heredity  : 

Doncaster,  L.  : Heredity  in  the  Light  of  Recent 
Research,  1911. 


CHAPTER  II 

EVOLUTION 

Evolution  may  be  defined  as  progress  involving  dif- 
ferentiation, an  ever-growing  complication  of  things 
which  accompanies  almost  aU  the  operations  of  Nature. 
The  idea  of  a differentiation  of  this  kind  may  be  en- 
forced by  a homely  and  quite  imaginary  illustration 
of  such  a process.  Imagine  the  proper  ingredients  of 
a plum  cake  to  be  very  finely  minced  and  intimately 
mixed  together,  so  as  to  form  a more  or  less  homo- 
geneous material.  Then,  if  by  any  means  the  separate 
particles  of  currants,  raisins,  peel,  and  so  forth,  could 
be  made  to  segregate  out  in  such  a way  as  to  give  rise 
to  the  ordinary  structure  of  this  pleasant  confection, 
we  should  have  arrived  at  the  structure  of  a plum  cake 
by  a process  of  evolution  involving  considerable  dif- 
ferentiation. 

The  progressive  increase  in  complexity  which  is 
Characteristic  of  so  many  natural  processes  is  in  great 
part  occasioned  by  the  fact  that  a single  ‘ cause  ’ is 
followed  as  a general  rule  by  more  than  one  ‘ effect.’ 
This  apparently  simple  circumstance  was  pointed  out 

22 


EVOLUTION  OF  MATTER 


23 


by  Herbert  Spencer,*  who  has  perhaps  done  more  than 
any  other  to  establish  and  emphasize  the  general  ap- 
plicability of  the  evolution  idea.  For  the  law  of  origin 
by  evolution  is  by  no  means  exclusively  confined  to 
the  method  of  coming  into  existence  of  the  species  of 
animals  and  plants.  On  the  contrary,  it  was  equally 
well  applied  by  Spencer  himself  to  describe  the  manner 
in  which  are  supposed  to  have  arisen  the  stars  and 
other  heavenly  bodies,  the  geological  strata  and  geo- 
graphical configuration  of  the  earth,  and  the  various 
gradations  of  human  society. 

The  discovery  that  certain  chemical  elements  exist 
which  are  themselves  not  immutable  has  been  made 
since  Spencer’s  time.  Quite  recently  ‘ the  phenomena 
of  radio-activity  have  forced  us  to  believe  that  radium 
is  passing  continuously  into  helium,’!  and  something 
more  than  a suspicion  has  been  aroused  that  radium 
is  itself  derived  from  uranium.  Thus  the  dreams  of 
the  alchemists  are  shown  to  have  been  not  wholly 
without  foundation,  for  the  probability  is  strong 

* Spencer  gives  the  following  illustration  : Regarding  the 
striking  together  of  two  bodies  as  a ' cause/  he  points  to  the 
following  possible  * effects  * : A sound  ; other  vibrations  or 
movements  in  the  surrounding  air  ; a disarrangement  of  the 
particles  of  the  two  bodies  in  the  neighbourhood  of  the  point 
of  collision  ; the  production  of  heat,  and  possibly  of  a spark — 
i,e,,  of  light. 

Two  words  in  this  sentence  are  placed  between  inverted 
commas,  to  indicate  that  they  are  used  in  a strictly  popular 
sense.  The  use  of  the  words  ' cause  ' and  * effect,'  though 
seldom  striciiy  scientific,  is  often  convenient,  and  if  used  with 
caution,  there  is  no  reason  why  they  should  lead  to  misunder- 
standing. See  Whetham,  ‘ The  Recent  Development  of 
Phyrical  Science/  chapter  i, 

t W'hctham. 


24 


EVOLUTION 


that  under  suitable  conditions  other  matter  may  be 
observed  to  behave  in  the  same  way  as  radium.  More 
than  this.  Professor  Sir  J,  J.  Thomson  has  been  able  to 
describe  the  atoms  of  the  elements  as  different  aggrega- 
tions of  a single  kind  of  corpuscles,  and  to  show  that 
a progressive  change  in  the  number  of  corpuscles 
making  up  the  atom  is  accompanied  by  a progressive 
alteration  in  the  properties  of  the  atom  itself,  so  that 
it  has  now  become  possible  to  establish  a theory  of 
the  evolution  of  the  chemical  elements  themselves. 

Passing  from  the  almost  immeasurably  small  to  the 
almost  immeasurably  great,  we  may  briefly  consider 
the  probable  mode  of  origin  of  the  solar  system  from 
an  extremely  diffuse  cloud  of  material  substance,  ac- 
cording to  the  famous  nebular  hypothesis  of  Laplace. 
By  a long-continued  process  of  contraction  under  the 
influence  of  gravity  the  nebular  substance  came  to  be 
of  varying  density,  and  acquired  a rotary  movement 
in  one  plane.  As  the  mass  continued  to  contract 
owing  to  the  mutual  attraction  of  its  particles,  the 
velocity  of  rotation  increased,  until  at  last  the  increas- 
ingly rapid  motion  of  the  outermost  ring  of  the  now 
lens-shaped  nebula  gave  rise  to  a centrifugal  force  great 
enough,  to  counteract  the  tendency  to  contraction,  and 
in  the  further  condensation  of  the  mass  this  ring  was 
left  behind.  The  ring  next  broke  down  at  one  point, 
and  contracting  on  itself  gave  rise  to  a single  spheroidal 
body  which  acquired  a movement  of  rotation  in  the 
same  direction  as  that  of  the  parent  nebula.  This 
body  was  the  outermost  planet  Neptune,  and  the  rest 
of  the  planets  were  produced  in  a similar  manneri 


EVOLUTION  IN  GEOLOGY 


25 


until  at  last  a central  mass  was  left,  and  this  became 
the  sun.  Satellites  were  thrown  off  from  several  of 
the  planets  just  in  the  same  way  as  the  planets  them- 
selves arose  from  the  original  nebula,  and  Saturn’s 
rings  are  pointed  to  as  showing  this  process  even  now 
in  course  of  operation. 

Such  a description  as  this  may  appear  fanciful  at 
first  sight,  but  it  was  worked  out  quantitatively  as 
well  as  qualitatively  by  its  author,  and  was  shown  to 
explain  in  detail  a multitude  of  phenomena.  Spencer 
points  out  that  when  we  have,  worked  out  by  one  of 
the  first  of  mathematicians,  a definite  theory  of  plane- 
tary evolution  based  on  established  mechanical  lawsi 
and  one  which  accounts  in  a satisfactory  way  for  all 
the  known  phenomena,  the  conclusion  that  the  solar 
system  really  did  arise  by  a process  of  evolution  is,  to 
say  the  least,  difficult  to  avoid. 

The  establishment  and  propagation  of  the  idea  that 
the  present  condition  of  the  earth’s  surface  arose 
through  a course  of  gradual  evolution,  by  the  agency 
of  such  processes  only  as  are  known  to  be  in  operation 
at  the  present  day,  is  the  great  contribution  of  Sir 
Charles  LyeU  to  the  science  of  geology.  We  may 
briefly  trace  the  evolution  of  the  idea  itself,  beginning 
with  the  speculations  of  Werner,  who,  from  observa- 
tions of  the  geological  formations  of  a limited  tract  of 
country,  came  to  the  conclusion  that  the  successive 
strata  were  precipitated  one  by  one  from  an  universal 
ocean.  Here  we  see  the  first  germ  of  the  idea  of 
evolution  embodied  in  the  notion  that  the  stratified 


26 


EVOLUTION 


rocks  came  into  existence  gradually  and  through  the 
operation  of  a supposed  natural  cause. 

A great  advance  upon  Werner’s  theory  was  made 
by  Hutton,  who,  observing  the  formation  of  strata 
at  the  present  day  from  the  sediment  washed  down 
by  rivers,  concluded  that  the  ancient  strata  were 
deposited  in  the  same  manner.  Since,  by  the  long 
continuation  of  this  process  the  continents  must 
gradually  become  reduced  to  the  level  of  the  sea, 
Hutton  supposed  that  at  long  intervals  of  time  the 
action  of  subterranean  heat  came  into  play,  and 
fresh  continents  were  upheaved,  a process  accom- 
panied by  the  outpouring  of  the  igneous  rocks, 
the  true  origin  of  which  he  had  duly  recognised. 
In  this  theory  a hypothetical  cause  stUl  survives,  since 
we  have  no  actual  experience  of  vast  upheavals  of  the 
kind  which  Hutton  supposed  to  have  taken  place. 
Lyell  showed  that  such  slight  changes  of  level  as  are 
known  to  be  in  progress  at  the  present  day,  especially 
in  association  with  the  phenomena  of  earthquakes, 
might,  if  continued  over  a long  series  of  ages,  give 
rise  to  the  necessary  amounts  of  elevation.  Lyell  also 
pointed  out  a number  of  subsidiary  causes  of  dis- 
integration and  deposition  of  strata  of  the  kind  which 
can  stUl  be  seen  in  operation  at  different  parts  of  the 
earth’s  surface.  At  the  present  time  it  is  sometimes 
thought  that  Lyell  went  a little  too  far  in  his  cham- 
pionship of  the  cause  of  uniformity.  LyeU  supposed 
that  the  agencies  which  may  now  be  everywhere  ob- 
served in  operation,  such  as  rain  and  rivers,  the  sea, 
volcanoes  and  earthquakes,  were  sufficient  to  account 


AGE  OF  THE  EARTH 


27 


for  all  the  phenomena  which  the  crust  of  the  earth 
exhibits.  It  is  now  more  generally  supposed  that  in 
very  early  times  forces  similar  in  kind  to  those  in 
action  at  the  present  day  may  have  exhibited  con- 
siderably greater  violence. 

To  produce  the  present  condition  of  the  surface  of 
the  earth  by  the  action,  gradually  accumulated,  of 
such  processes  of  denudation  and  upheaval  as  are  now 
going  on  around  us,  vast  periods  of  time  are  clearly 
necessary.  The  early  evolutionists,  having  once  got 
rid  of  the  idea  that  the  date  given  by  Bishop  Usher 
as  that  of  the  creation  of  the  world  is  a necessary  and 
integral  part  of  religion,  immediately  allowed  their 
imaginations  to  run  riot  with  regard  to  the  amount 
of  time  at  their  disposal.  Since  this  question  of  the 
extent  of  geological  time  has  an  important  bearing  on 
the  problem  of  organic  as  well  as  upon  that  of  inorganic 
evolution,  it  will  be  well  to  pay  some  attention  to 
more  recent  views  upon  the  subject. 

Some  years  ago  the  generous  ideas  of  biologists  as 
well  as  of  geologists  were  to  a great  extent  shattered 
by  the  calculations  of  Lord  Kelvin.  These  were  based 
upon  three  separate  sets  of  data,  which  we  may  enume- 
rate without  entering  into  a lengthy  explanation  of 
the  calculations  involved.  The  evidence  made  use  of 
consisted  of  (i)  the  rate  of  the  earth’s  rotation,  as 
affected  by  tidal  retardation ; (2)  the  rate  of  secular 
cooling  of  the  earth,  as  deduced  from  the  rate  at  which 
the  temperature  of  the  earth’s  crust  rises  on  passing 
inward  from  the  surface  ; and  (3)  the  rate  of  cooling  of 
the  sun  by  radiation.  The  three  calculations  were 


28 


EVOLUTION 


found  to  show  a very  fair  measure  of  agreements  and 
they  led  to  the  conclusion  that  considerably  less  than 
a hundred  million  years  has  elapsed  since  the  first  for- 
mation of  seas  upon  this  planet,  an  event  which  must 
have  preceded  the  possibility  of  aqueous  geological 
action  and  the  existence  of  living  organisms 

Allowing  for  the  circumstance  that  geological  pro- 
cesses may  have  gone  forward  with  considerably 
greater  rapidity  during  the  earlier  periods  of  the 
earth’s  history  than  is  the  case  at  the  present  day,  the 
time  thus  allowed  by  the  physicist  is  generally  regarded 
by  geologists  as  too  little.  Reckoning  from  the  known 
rate  of  denudation,  which  is,  of  course,  the  same  as 
the  rate  at  which  the  same  material  is  deposited 
beneath  the  sea,  Geikie,  who  admitted,  however,  that 
such  data  are  only  of  a very  rough  description, 
concluded  that  the  space  of  a hundred  nuUion  years 
would  afiord  sufficient  time  for  the  laying  down 
of  the  known  aqueous  strata.  But  there  can  be 
little  doubt  that  the  lower  metamorphosed  rocks 
represent  a much  longer  period  of  time  than 
the  primary,  secondary,  and  tertiary  epochs  added 
together ; consequently,  the  respective  estimates  of 
Lord  Kelvin  and  the  geologists  appear  to  be  contra- 
dictory. The  recent  discovery  of  the  enormous  quan- 
tities of  energy  stored  up  in  radio-active  substances 
introduces  a serious  modification  into  the  mathematical 
argument  from  astronomical  data,  and  Sir  George 
Darwin  ‘ sees  no  reason  for  doubting  the  possibility 
of  augmenting  the  estimates  of  solar  heat,  as  derived 
from  the  theory  of  gravitation,  by  some  such  factor 


ORGANIC  EVOLUTION 


29 


as  ten  or  twenty,’  on  the  supposition  that  a consider- 
able proportion  of  the  sun’s  substance  was  made  up  of 
radio-active  material. 

The  above  remarks  may  serve  to  illustrate  the  im- 
portance of  the  theory  of  evolution  as  applied  to  the 
two  sciences  of  astronomy  and  geology.  We  pass  next 
to  a brief  historical  consideration  of  the  development 
of  the  evolution  theory  as  a method  of  describing  the 
origin  of  the  species  of  animals  and  plants. 

The  views  of  the  ancient  Greeks  cannot  be  said  to 
have  much  more  than  a purely  speculative  interest. 
Some  rudiments  of  the  idea  of  evolution  have  been 
attributed  to  Empedocles  as  well  as  to  several  other 
early  writers,  and  in  the  writings  of  Aristotle,  for 
whom  the  too  great  faith  of  his  successors  for  many 
ages  has  been  followed  by  a somewhat  unmerited 
degree  of  contempt  in  modern  times,  we  find  that  the 
evolution  idea  had  reached  quite  a respectable  degree 
of  development. 

In  the  Middle  Ages  the  adoption  of  the  Jewish  cos- 
mogony by  the  Christian  Churches  effectually  annihi- 
lated all  useful  thought  upon  the  subject  of  speciesj 
since  the  hypothesis  of  separate  creation  affords  no 
scope  for  further  speculation  or  experiment,  and  it  is 
not  until  the  end  of  the  seventeenth  century  that  we 
find  thoughtful  men  beginning  to  struggle  against  the 
ecclesiastical  bondage.  Thus  Erasmus  Darwin  de- 
rived the  idea  of  generation  rather  than  creation  of 
the  world  from  David  Hume,  and  himself  waxes 
enthusiastic  over  the  thought  : 

‘ That  is,  it  (the  world)  might  have  been  gradually 


30 


EVOLUTION 


produced  from  very  small  beginnings,  increasing  by 
the  activity  of  its  inherent  principles,  rather  than  by 
a sudden  evolution  of  the  whole  by  the  almighty  fire. 
What  a magnificent  idea  of  the  infinite  power  of  The 
Great  Architect  ! The  Cause  of  causes.  Parent  of 
parents.  Ens  entium.’’ 

De  Maillet,  writing  in  1735,  showed  a definite  idea 
of  the  production  of  existing  species  by  the  modifica- 
tion of  their  predecessors.  At  the  beginning  of  the 
nineteenth  century  similar  speculations  were  published 
by  Goethe  and  by  Treviranus,  and  the  latter  was  the 
first  to  apply  the  term  ‘ biology  ’ to  the  science  of  the 
phenomena  of  life.  Lamarck  about  the  same  time  pro- 
vided a definite  theory  as  to  the  method  by  which  the 
modification  of  species  takes  place. 

Before  discussing  Lamarck’s  hypothesis  and  the 
alternative  theories  more  recently  proposed,  it  will  be 
well  to  pass  in  review  the  evidence  upon  which  is 
based  our  belief  that  the  species  of  animals  and  plants 
have  arisen  through  the  modification  of  pre-existing 
species^  and  to  show  that  the  greater  part  of  this  evi- 
dence is  quite  independent  of  any  views  which  we  may 
adopt  as  to  the  actual  method  by  which  a particular 
species  came  into  existence.  And  in  the  first  place  we 
may  point  out  the  entire  absence  of  any  evidence,  direct 
or  indirect,  in  favoiir  of  the  alternative  supposition  of 
a special  creation  of  each  separate  species. 

The  evidence  for  evolution  falls  naturally  into  a 
number  of  fairly  well  defined  sections ; these  have 
been  so  admirably  summarized  by  Huxley  in  his 
essay  on  ‘ Evolution  in  Biology,’  that  we  cannot  well 


EVIDENCES  OF  ORGANIC  EVOLUTION  31 


do  better  than  recapitulate,  with  only  slight  modifi- 
cations, the  arguments  there  given  : 

1.  The  Gradation  of  Organisms. — Both  in  the 
animal  and  vegetable  kingdoms  we  may  trace,  in  spite 
of  certain  gaps,  a long  series  of  gradations  in  com- 
plexity of  structure,  so  that  between  the  simplest  and 
the  most  complicated  of  living  things  a great  number 
of  intermediate  stages  are  to  be  found.  When  we 
pass  to  the  lower  end  of  the  scale  in  either  case,  we 
come  upon  a group  of  creatures  of  comparatively 
simple  organization.  Among  them  we  find  members 
with  regard  to  which  we  cannot  definitely  say  that 
they  are  either  animals  or  plants.  Moreover,  these 
unicellular  organisms  resemble  in  many  ways  the 
egg-cell  from  which  every  individual  among  the  higher 
animals  and  plants  originates. 

It  is  true  that  we  now  know  it  to  be  quite  impossible 
to  dispose  all  the  members  of  the  animal  kingdom  in 
a single  linear  series,  such  as  was  formerly  suggested, 
passing  in  orderly  sequence  from  the  amoeba  up  to 
man.  ‘ Instead  of  regarding  living  things  as  capable 
of  arrangement  in  one  series,  like  the  steps  of  a ladder, 
the  results  of  modern  investigation  compel  us  to  dis- 
pose them  as  if  they  were  the  twigs  and  branches  of  a 
tree.  The  ends  of  the  twigs  represent  individuals, 
the  smallest  groups  of  twigs  species,  larger  groups 
genera,  until  we  arrive  at  the  source  of  all  these 
ramifications  of  the  main  branch,  which  is  represented 
by  a common  plan  of  structure.’ 

2.  Embryology. — All  the  members  of  a particular 
group  of  animals  or  plants  as  a rule  resemble  one 


32 


EVOLUTION 


another  more  closely  in  the  early  stages  of  their  indi- 
vidual development  than  they  do  in  the  adult  condi- 
tion, and  in  the  earliest  stages  of  all  they  are  often 
indistinguishable.  These  facts  are  explained  if  we 
suppose  that  such  individuals  have  a common  origin, 
that  they  are  descended  from  a common  ancestor,  and 
that  traces  of  their  pedigree  are  still  to  be  observed  in 
the  developmental  stages  through  which  each  one 
passes.  We  do  not  find  a complete  parallelism  be- 
tween the  development  of  the  individual  and  the 
history  of  the  race,  nor  should  we  expect  to  do  so, 
since  embryonic  as  well  as  adult  stages  may  be  modified 
in  the  course  of  evolution  ; what  we  should  expect  is 
a more  or  less  vague  historical  sketch,  and  this  is  what 
is  usually  found  remaining. 

‘ It  is  not  true,  for  example,  that  a fish  is  a reptile 
arrested  in  its  development,  or  that  a reptile  was  ever 
a fish  ; but  it  is  true  that  the  reptile  embryo,  at  one 
stage  of  its  development,  is  an  organism  which,  if  it 
had  an  independent  existence,  must  be  classified 
among  fishes  ; and  all  the  organs  of  the  reptile  pass,  in 
the  course  of  their  development,  through  conditions 
which  are  closely  analogous  to  those  which  are  per- 
manent in  some  fishes.’ 

3..  Morphology. — On  comparing  together  the  dif- 
ferent members  of  one  of  the  great  groups  or  classes 
of  animals  or  plants,  we  find  the  same  fundamental 
plan  of  organization  running  through  all  of  them. 
Series  of  corresponding  organs  are  often  to  be  made 
out  which  are  built  upon  the  same  general  scheme^ 
although  their  functions  may  be  quite  dissimilar ; so 


EVIDENCES  OF  ORGANIC  EVOLUTION  33 


that,  for  instance,  in  the  hand  of  a man^  the  paw  of  a 
dog,  the  wing  of  a bat,  and  the  paddle  of  a whale, 
almost  identically  the  same  series  of  bones  can  be 
traced.  An  obvious  explanation  is  to  be  found  in  the 
supposition  that  these  parts  have  arisen  by  the 
divergent  modification  of  parts  which  were  originally 
identical. 

4.  Geographical  Distribution.  — Observation 
shows  that  groups  of  closely  allied  creatures  are  often 
found  living  in  neighbouring  districts,  and  that  when 
such  a barrier  as  an  ocean  or  a range  of  lofty  moun- 
tains is  passed  an  entirely  new  fauna  and  flora  are 
usually  to  be  met  with.  These  facts  may  be  explained 
by  the  hypothesis  that  allied  groups  of  species  origi- 
nated by  a process  of  descent  in  the  same  countries 
which  they  now  inhabit,  and  they  can  be  explained 
by  no  other  known  hypothesis.  The  alternative  sup- 
position that  each  species  was  specially  created  and 
placed  in  the  locality  in  which  it  was  best  adapted 
to  dwell  is  singularly  in  disagreement  with  the  well- 
known  facts  that  animals  and  plants  transported  into 
entirely  new  regions  often  thrive  better  than  in  their 
original  homes.  The  examples  of  rabbits  in  Australia 
and  of  cardoons  and  thistles  on  the  Pampas  of  La 
Plata  are  familiar  to  all  from  the  writings  of  Darwin. 

5.  The  Geological  Succession  of  Organisms. — 
The  general  facts  regarding  the  distribution  of  allied 
species  of  animals  and  plants  in  time  point  in  pre- 
cisely the  same  direction  as  those  relating  to  their 
distribution  in  space.  In  a few  cases,  notably  in  that 
of  the  extinct  horse  of  North  America,  a long  chain  of 

3 


34 


EVOLUTION 


possibly  ancestral  types  has  been  found  leading  back 
to  a remote  and  very  different  progenitor.  This  sup- 
posed ancestor  of  the  horse  was  a creature  little  larger 
than  a moderate-sized  dog.  It  had  four  separate  toes 
to  each  fore-limb,  and  three  to  each  hind-limb,  and  its 
teeth  were  much  simpler  and  less  specialized  than 
those  of  existing  horses.  We  say  possibly  ancestral, 
advisedly.  Geological  evidence  is  naturally  insufficient 
to  establish  the  actual  relationship  of  the  series  of 
types  which  has  been  described,  and  Professor 
Sedgwick  has  criticized  the  view  that  this  series 
of  forms  constitutes  a demonstrative  historical  proof 
of  the  doctrine  of  organic  evolution.  It  is,  therefore, 
preferable  to  claim  this  group  of  fossils  as  an  illustra- 
tion of  the  possible  geological  ancestry  of  an  existing 
species  rather  than  as  affording  concrete  proof  of  an 
actual  pedigree.  Even  with  this  reservation,  we  claim 
that  such  a series  constitutes  a valuable  collateral  proof 
in  favour  of  evolution.  The  general  distribution  of 
organisms  throughout  the  geological  strata  agrees 
moreover,  in  a remarkable  way  with  what  is  to  be 
expected  on  the  evolution  theory. 

6.  Changes  under  Domestication. — Among  do- 
mesticated animals  and  plants  we  know  of  numerous 
cases  in  which  the  actual  origin  of  new  forms  has  been 
observed.  These  have  often  differed  from  their  pre- 
decessors by  amounts  quite  comparable  with  the  dif- 
ferences by  which  natural  species  or  even  genera  are 
separated.  A notable  example  of  this  process  is  afforded 
by  the  numerous  breeds  of  pigeons  known  to  have  arisen 
under  domestication  from  a single  wild  species.  We 


EVIDENCES  OF  ORGANIC  EVOLUTION  35 


have  no  reason  whatever  for  supposing  that  domesti- 
cated species  are  more  mutable  than  wild  species,  and 
there  is  consequently  every  reason  to  beheve  that 
changes  of  a similar  character  take  place  in  Nature. 

Mr.  Alfred  Russel  Wallace  has  quoted  with  approval 
Sir  W.  Thiselton  Dyer’s  criticism  that  if  there  is  an 
equal  chance  of  the  occurrence  or  origin  of  new  forms 
in  nature  and  under  cultivation,  then  their  appearance 
— i.e.,  their  survival  until  a stage  at  which  they  can 
be  readily  recognized  as  distinct  from  the  original  type 
— should  be  more  frequent  in  nature  than  in  cultiva- 
tion, because  the  former  has  a larger  population  to 
work  with.  The  reply  to  this  argument  is  obvious. 
In  the  first  place,  the  much  greater  facilities  for  obser- 
vation under  cultivation  may  fairly  be  set  against  the 
greater  numbers  stated  to  exist  in  nature  ; but,  in  the 
second  place,  Mr.  Wallace  may  well  be  challenged  to 
cite  a natural  species  of  which  a larger  number  of 
individuals  has  passed  under  man’s  observation  than 
is  the  case  with  cultivated  wheat,  for  example.  But  a 
third  line  of  argument  is  much  more  conclusive  than 
either  of  these.  The  modifications  which  occur  under 
cultivation  are  in  most  cases  decidedly  weakly  as  com- 
pared with  the  original  forms,  as  every  gardener  knows 
to  his  cost.  They  are  only  enabled  to  survive  to  a 
recognizable  stage,  because  cultivation  consists  in  the 
removal  of  competition ; all  are  given  an  equal  chance. 
This  is  not  so  in  nature.  There,  competition  (according 
to  the  exponents  of  the  WaUacian  doctrine)  is  so  intense 
that  even  very  slight  variations  may  determine  success 
or  failure.  According  to  the  doctrine  of  natural  selec- 

3— a 


36 


EVOLUTION 


tion,  then,  decidedly  weakly  specimens,  if  they  occurred 
in  nature,  would  have  practically  no  chance  of  sur- 
vival, and  would  consequently  never  be  seen.  This 
attempt  to  undermine  one  of  the  strongest  evidences 
of  organic  evolution,  therefore,  falls  to  the  ground. 

7.  The  Observed  Facts  of  Mutation.  — As  a 
matter  of  fact,  novel  types  are  seen  in  nature  not  in- 
frequently, and  are  specially  common  in  some  groups 
of  plants,  as  Mr.  C.  T.  Druery  has  sho\vn  for  the  case 
of  the  British  ferns ; and  isolated  specimens  of  par- 
ticular wild  species  belonging  to  other  families  are  fre- 
quently found,  which,  if  they  had  occurred  as  con- 
stant features  of  a considerable  group  of  individuals, 
would  afford  a basis  for  the  description  of  a new  species. 
The  study  of  mutation  will,  however,  require  a special 
chapter  of  its  own. 

We  see,  therefore,  that  the  evidence  in  favour  of  the 
existing  species  of  animals  and  plants,  having  arisen 
by  a process  of  evolution,  is  of  a most  ample  and  con- 
vincing kind.  The  theory  of  organic  evolution  is, 
however,  incomplete  until  we  have  arrived  at  a true 
account  of  the  method  or  methods  by  which  new 
species  arise  from  old  ones.  The  earliest  definite 
explanation,  as  already  stated,  was  that  given  by 
Lamarck,  and  we  may  next  proceed  to  consider  the 
Lamarckian  theory  of  the  origin  of  species. 

Earlier  writers  had  already  supposed  that  species 
became  modified  through  the  action  of  the  external 
conditions  to  which  they  were  exposed.  Lamarck  laid 
special  stress  upon  the  observed  facts  that  the  organs 
of  individuals  become  increased  and  developed  through 


THE  THEORY  OF  LAMARCK 


37 


use,  and  that  disuse  is  followed  by  a dwindling  and 
loss  of  the  power  of  action.  By  the  inherited  effects 
of  use  and  disuse,  and  of  modifications  caused  by  ex- 
ternal conditions,  Lamarck  supposed  all  evolution  of 
species  to  have  come  about. 

Reference  has  already  been  made  to  Lamarck’s  de- 
scription of  the  method  of  origin  of  the  characteristic 
form  of  snakes,  owing  to  the  endeavours  of  the  snakes’ 
ancestors  to  creep  through  narrow  passages.  Lamarck 
was  quite  consistent  inasmuch  as  he  explained  the 
different  types  which  have  arisen  among  domesticated 
species  by  the  same  theory  as  he  applied  to  the  origin 
of  species  in  a state  of  nature.  Thus  he  supposed  the 
differences  between  race-horses  and  heavy  cart-horses 
to  be  the  direct  result  of  the  different  kinds  of  enforced 
exercise  to  which  the  ancestors  of  these  races  were 
respectively  subjected.  Similarly,  all  the  different 
breeds  of  dogs  were  supposed  to  have  arisen  owing  to 
the  different  habits  which  the  various  successors  of  the 
first  domesticated  dogs  acquired,  small  changes  being 
accumulated  by  inheritance  in  each  successive  genera- 
tion. 

Turning  now  to  species  in  a state  of  nature,  the  case 
of  the  giraffe  is  one  of  those  most  often  quoted. 
Lamarck  supposed  a comparatively  short  - necked 
ancestor  of  the  giraffes  to  have  taken  up  the  habit  of 
browsing  upon  the  leaves  of  trees,  owing  to  the  diffi- 
culty of  obtaining  other  food  in  an  arid  region.  In 
order  to  obtain  their  new  food  the  animals  were  obliged 
to  be  continually  stretching  upward,  and  the  effort  to 
elongate  their  necks  was  attended  with  some  small 


38 


EVOLUTION 


measure  of  success  in  each  individual.  This  increase, 
being  accumulated  by  inheritance  in  every  succeeding 
generation,  ultimately  led  to  the  great  stature  exhibited 
by  the  giraffes  of  the  present  day. 

The  stilt-like  legs  of  many  wading  birds  were  ascribed 
by  Lamarck  to  the  result  of  the  continued  attempts  of 
ancestors  which  had  shorter  extremities  to  obtain  their 
food  in  shallow  water  without  wetting  their  feathers. 
The  long-continued  endeavours  of  these  birds  to  stretch 
and  elongate  their  legs  had  the  same  effect  as  the 
similar  efforts  made  by  the  ancestors  of  the  giraffes. 
It  has  been  suggested,  however,  by  a critic  of  Lamarck's 
position  that  such  birds  would  be  likely  to  eschew  fish 
dinners  long  before  any  notable  increase  in  the  length 
of  their  legs  was  arrived  at. 

If  some  of  the  above  cases  appear  a little  ludicrous, 
there  are  other  instances  in  which  the  Lamarckian 
hypothesis  seems  to  afford  a perfectly  adequate  and 
natural  explanation.  Such  a case  is  afforded  by  the 
family  of  the  flat  fisheSj  including  such  well-known 
species  as  the  sole  and  plaice.  In  the  adult  condition 
these  fishes  lie  flat  on  one  side ; and  during  their 
development  from  the  young  condition,  that  eye  which, 
if  it  remained  in  its  original  position,  would  look 
directly  downwards  travels  round  the  head  until  it 
comes  to  lie  quite  upon  the  upper  surface.  As  Darwin 
pointed  out,  agreeing  in  this  with  Mivart,  a sudden 
spontaneous  transformation  in  the  position  of  the  eye 
is  hardly  conceivable,  and  it  is  equally  impossible  to 
explain  the  origin  of  this  remarkable  feature  by  the 
action  of  natural  selection,  because  a slight  change 


THE  THEORY  OF  LAMARCK 


39 


in  the  position  of  the  eye  could  be  of  no  advantage 
so  long  as  this  organ  remained  upon  the  under  surface. 
The  very  young  fish,  whilst  still  symmetrical,  are 
known  sometimes  to  fall  upon  one  side,  and  when  in 
this  position  to  twist  the  lower  eye  forcibly  upwards. 
Darwin  himself  therefore  supposed  that  the  origin  of 
the  adult  structure  is  to  be  attributed  to  the  inherited 
effect  of  efforts  of  this  kind. 

The  interest  of  the  last  case  lies  in  the  fact  that  it 
relates  to  a structure,  the  origin  of  which  does  not 
appear  explicable  on  the  theory  of  natural  selection ; 
its  bearing  will  therefore  be  better  understood  when 
we  come  to  discuss  that  theory  in  the  next  chapter. 

The  inherited  effects  of  voluntary  striving  can  clearly 
have  no  application  to  the  case  of  plants.  Lamarck 
therefore  supposed  that  evolution  in  the  vegetable 
kingdom  had  taken  place  entirely  through  the  action  of 
external  agencies  upon  plants.  The  soil,  for  examplej 
in  which  a plant  grows  has  a direct  influence  upon  its 
form.  Altitude,  moisture,  heat,  and  light  are  other 
important  factors^  and  the  effect  of  their  influence  upon 
the  plant  was  supposed  by  Lamarck  to  be  inherited. 
The  shape  of  irregular  flowers  was  regarded  as  having 
been  directly  caused  by  the  strains  and  pressures 
occasioned  by  bees  and  other  insects  whilst  making 
their  visits  in  search  of  honey  or  pollen. 

Lamarck’s  theory  turns  entirely  upon  the  question 
whether  acquired  characters  are  inherited,*  and  if  soi 
to  what  extent;  siifee,  if  such  inheritance  is  shown  to 
be  extremely  slight,  the  cause,  though  a true  one, 
may  be  insufficient  to  explain  the  effects  attributed 


40 


EVOLUTION 


to  it.  Now,  theories  of  heredity  apart,  and  leaving 
aside  the  results  of  minute  observations  which  had 
not  been  made  in  Lamarck’s  time,  the  natural  supposi- 
tion undoubtedly  is  that  acquired  characters  are 
inherited  just  as  much  as  any  others.  Given  the  ob- 
served fact  that  offspring  resemble  their  parents  more 
closely  than  they  do  other  members  of  the  same  species,' 
it  is  natural  to  believe  that  the  child  will  take  after 
the  forms  exhibited  by  its  parents  at  the  time  of  its 
conception  rather  than  after  those  shown  by  them  at 
any  previous  period  of  their  lives.  This  seems  to  be 
the  natural  view  in  the  absence  of  any  other  evidence 
for  or  against,  and  so  accurate  a thinker  as  Herbert 
Spencer,  writing  before  the  publication  of  the  ‘ Origin 
of  Species,’  regarded  the  term  inheritance  as  neces- 
sarily implying  inheritance  of  this  particular  kind. 
For  this  rfeason  it  has  sometimes  been  thought  that 
Darwin  scarcely  accorded  to  Lamarck  the  appreciation 
which  he  deserved ; and  yet  Darwin  himself  fell  back 
upon  the  Lamarckian  explanation  on  the  few  occasions 
when  natural  selection  seemed  to  have  failed  him. 

When,  however,  we  come  to  know  more  of  the  actual 
facts  of  sexual  generation,  we  find  that  it  is  very 
difficult  j if  not  impossible,  to  imagine  any  kind  of 
mechanism  by  which  this  supposed  transmission  of 
acquired  modifications  can  take  place.  We  shall  defer 
the  further  discussion  of  this  subject,  as  well  as  the 
question  of  the  existence  of  direct  and  other  evidence 
of  use  inheritance,  until  the  latter  half  of  the  next 
chapter,;  where  we  shall  refer  briefly  to  the  contro- 


THE  THEORY  OF  LAMARCK  41 

versy  upon  these  subjects  which  followed  the  estab- 
lishment of  the  principle  of  natural  selection. 

BIBLIOGRAPHY. 

Whetham,  W.  C.  D.  : The  Recent  Development  of  Physical 
Science,  1904. 

Spencer,  Herbert  : Essays — Scientific,  Political,  and  Specu- 
lative, 1868. 

Huxley,  T.  H.  : Collected  Essays,  vol.  ii.,  1899. 

Romanes,  G.  J.  : Darwin  and  After  Darwin,  vol.  i.,  1897. 
Lamarck,  J.  B.  : Philosophie  Zoologique,  1809. 

Lyell,  Sir  Charles,  Principles  of  Geology,  eleventh  edition, 
1872. 

Butler,  S.  : Evolution  Old  and  New,  1879. 


CHAPTER  III 


THE  THEORY  OF  NATURAL  SELECTION 

In  1813  a communication  was  read  before  the  Royal 
Society  by  Dr.  W.  C.  Wells  upon  the  differentiation 
which  exists  between  certain  races  of  mankind.  In 
Dr.  Wells’s  paper  this  differentiation  was  explained 
from  the  facts  thats  since  no  two  individuals  are  alike, 
some  would  be  better  fitted  than  others  to  resist  the 
diseases  proper  to  a particular  country,  and  would 
consequently  tend  to  survive,  whilst  their  less  fortunate 
neighbours  would  perish  in  greater  numbers.  Wells 
supposed  the  dark  races  of  mankind  to  be  better 
adapted  to  warm  climates  than  white  races  are,  and 
he  thus  applied  to  the  particular  case  of  the  human 
species  the  true  Darwinian  principle  of  a gradual 
evolution  through  the  survival  of  the  fittest. 

A similar  view  was  applied  to  the  origin  of  species 
in  general  by  Patrick  Matthews  in  a book  on  naval 
timber  and  arboriculture  published  in  1831. 

Both  these  works  were  unknown  to  Darwin  at  the 
time  of  the  first  publication  of  the  ‘ Origin  of  Species,’ 
and  it  is  quite  unnecessary  to  point  out  that  their 
existence  does  not  in  the  least  prejudice  the  value  or 
originality  of  that  great  work.  Their  interest  at  the 

42 


HISTORICAL 


43 


present  time  is  merely  historicaiii  as  showing  the 
direction  in  which  thought  was  tending  in  the  earlier 
half  of  the  nineteenth  century. 

Before  the  ‘ Origin  of  Species  ’ was  published, 

A.  R.  Wallace  communicated  to  Darwin  a paper  in 
which  the  bearing  of  the  same  idea  was  worked  out  at 
some  length,,  and  this  paper  was  read,  together  with 
an  abstract  of  Darwin’s  own  views,  at  a meeting  of 
the  Linnean  Society  in  July,  1858. 

With  this  notice  of  other  claimants  to  the  idea  of  . 
natural  selection  we  may  proceed  to  give  an  account 
of  the  theory  as  it  is  developed  in  the  earlier  chapters 
of  the  ‘ Origin  of  Species.’ 

We  must  first  glance  at  Darwin’s  method  of  using 
the  term  variation.  Darwin  applied  this  term  to  every 
kind  of  difference  which  is  found  to  occur  between 
parents  and  their  offspring,  or  between  members  of 
the  same  family,  no  matter  whether  these  differences 
were  great  or  small.  It  has  since  been  shown  that  a 
number  of  quite  distinct  phenomena  were  in  this  way 
regarded  from  a single  standpoint,  without  a proper 
discrimination  being  made  between  them.  But  the 
differences  between  continuous  and  discontinuous 
variation,  quantitative  and  qualitative  variation,  and 
the  rest,  were  not  pointed  out  until  long  subsequent  to 
1859.  Thus,  beyond  recognising  a distinction  between 
sports  and  individual  differences,  and  attaching  greater 
weight  to  the  latter  kind  of  changes,  as  being  those 
which  chiefly  led  to  the  origin  of  new  species,  Darwin 
made  no  further  analysis  of  the  facts  of  variation,  but 
accepted  all  sorts  of  differences  between  individuals  as 


44  THE  THEORY  OF  NATURAL  SELECTION 


affording  the  material  upon  which  natural  selection 
might  be  expected  to  operate. 

The  idea  that  a selective  influence  exists  in  Nature 
arose  from  a study  of  the  remarkable  effects  produced 
in  the  case  of  domestic  animals  and  plants  by  the  action 
of  artificial  selection.  Darwin  seems,  however,  to  have 
been  a little  credulous  in  accepting  the  statements  of 
certain  breeders  with  regard  to  their  power  of  producing 
any  desired  new  type  to  order.  Now  that  scientific 
men  are  themselves  beginning  to  make  experiments 
in  breeding,  with  the  check  of  exact  records  to  act 
as  a drag  upon  the  exuberance  of  the  imagination, 
they  are  becoming  somewhat  sceptical  as  to  the  mystic 
and  almost  miraculous  powers  attributed  to  the  old- 
fashioned  breeders,  though,  indeed,  Mr.  Liither  Bur- 
bank would  seem  to  be  a survival  from  the  period 
we  speak  of,  if  the  statements  of  his  recent  enthusiastic 
biographer  are  to  be  credited.*  Less  gifted  but  more 
methodical  observers  find  that  they  have  no  creative 
powers  worth  speaking  of,  and  that  all  they  can  do  is 
to  keep  a sharp  look-out  for  the  novelties  which  Nature 
may  send  them. 

Selection,  whether  natural  or  artificial,  can  indeed 
of  itself  have  no  power  in  the  direction  of  creating 
anything  new  ; its  influence  is  destructive  or  preserva- 
tive, but  nothing  more  than  this.  The  breeder  keeps 
the  new  forms  which  take  his  fancy,  and  destroys  the 
rest ; that  is  the  whole  story. 

♦ Harwood,  ‘ New  Creations  in  Plant  Life.'  Mr.  Burbank 
certainly  seems  to  have  a really  wonderful  instinct  for  the  dis- 
covery of  curious  and  useful  novelties. 


ARTIFICIAL  SELECTION 


45 


Yet  a remarkable  number  of  new  kinds  of  creatures 
are  known  to  have  arisen  in  this  way,  and  their  diver- 
sity is  no  less  astonishing,  as  a visit  to  any  great  show 
of  domestic  plants  or  animals  will  at  once  demonstrate. 
Here  may  be  seen  varieties  of  pigeons,  for  example, 
like  the  carrier,  pouter,  fantail,  and  tumbler,  which, 
if  they  were  found  existing  in  a wild  condition,  would 
be  placed  in  separate  genera  by  any  ornithologist. 
The  domestic  races  of  fowls,  dogs,  horses,  sheep,  and 
cattle  show  scarcely  less  divergence,  and  modifications 
no  less  remarkable  have  been  perpetuated  in  the  case 
of  many  cultivated  species  of  plants.  Whilst  these 
types  have  survived,  being  deliberately  preserved  on 
account  of  their  use  or  beauty  or  curious  appearance,  a 
still  greater  number  have  doubtless  been  exterminated, 
either  because  they  did  not  attract  the  breeder’s 
favourable  attention,  or  on  account  of  their  having 
passed  out  of  fashion. 

Darwin  sought  in  Nature  a substitute  for  the  baleful 
judgment  of  the  breeder,  and  found  it  in  an  extension 
of  the  Malthusian  doctrine  to  organic  beings  in  general. 
The  idea  which  is  identified  with  this  expression 
did  not,  however,  originate  with  Malthus,  nor  does 
that  author  claim  it  as  his  own,  as  the  following  extract 
from  the  first  chapter  of  the  ‘ Essay  on  Population ' 
will  show  : 

‘ It  is  observed  by  Dr.  Franklin  that  there  is  no  bound 
to  the  prolific  nature  of  animals  and  plants  but  what 
is  made  by  their  crowding  and  interfering  with  each 
other’s  means  of  subsistence.  Were  the  face  of  the 
earth,  he  says,  vacant  of  other  plants,  it  might  be 


46  THE  THEORY  OF  NATURAL  SELECTION 


gradually  sowed  and  overspread  with  one  kind  only, 
as,  for  instance,  with  fennel ; and  were  it  empty  of  other 
inhabitants,  it  might  in  a few  ages  be  replenished  from 
one  nation  only,  as,  for  instance,  with  Englishmen.’ 

Malthus’  ‘ Essay  ’ was  first  published  in  1798,  and 
was  subsequently  much  enlarged.  Its  author  proved 
incontrovertibly,  by  a survey  of  facts  gathered  from 
almost  all  the  countries  of  the  world,  that  human  popu- 
lation tends  to  increase  in  a geometrical  ratio,  and 
that,  consequently,  after  a time,  the  less  gifted  classes 
of  any  community  are  bound  to  suffer  from  a stress  of 
poverty,  only  partly  relieved  by  a high  infant  mortality, 
periodic  famines,  and  similar  factors,  or  in  less  civilized 
countries  by  infanticide  and  other  artificial  checks. 

Among  animals  and  plants  in  a state  of  nature  the 
rate  of  increase  is  often  very  much  greater  than  in  the 
case  of  the  human  family,  and  even  where  it  is  not  so, 
unchecked  breeding  would  in  a comparatively  few  years 
lead  to  the  overpeopling  of  the  earth  with  the  de- 
scendants of  a single  pair.  As  an  example  of  the  rate 
of  increase  shown  by  a wild  species,  we  may  consider 
the  case  of  the  elephant,  instanced  by  Darwin  himself, 
since  this  is  regarded  as  being  one  of  the  slowest 
breeders  among  all  known  animals.  Darwin  assumes 
that  the  elephant  begins  breeding  at  thirty  years,  and 
continues  to  do  so  until  it  reaches  the  age  of  ninety, 
bringing  forth  six  young  in  the  interval,  and  surviving 
to  the  age  of  a hundred.  Then,  if  there  were  no 
casualties,  he  calculates  that  after  from  740  to  750 
years  there  would  be  nearly  nineteen  million  elephants 
alive  descended  from  the  first  pair. 


RATE  OF  INCREASE 


47 


Let  us  also  consider  the  case  of  a minute  rapidly 
breeding  animal  of  a typical  kind.  My  friend  Mr. 
R.  C.  Punnett  has  recently  been  engaged  upon  an 
experiment  which  involved  the  breeding  of  rotifers, 
a kind  of  animal  barely  visible  to  the  naked  eye. 
They  were  bred  for  sixty-seven  generations,  and 
each  individual  produced  on  the  average  thirty  eggs. 
The  whole  experiment  occupied  less  than  a year,  yet 
Mr.  Punnett  calculated  that  if  he  had  been  able  to 
rear  all  the  animals  which*  at  this  rate  of  breeding,  for 
this  number  of  generations,  were  theoretically  obtain- 
able, he  would  have  become  the  possessor  of  a solid 
sphere  of  organic  material  with  a radius  greater  than 
the  probable  limits  of  the  known  universe. 

This  geometrical  rate  of  increase  is  common  in  a 
greater  or  less  degree  to  all  living  organisms.  Since 
the  space  and  food-supply  available  for  the  support  of 
any  species  has  no  corresponding  tendency  to  in- 
crease, it  follows  that  a large  proportion  of  the 
individuals  born  must  perish  before  they  reach  the 
adult  state,  or  at  least  without  producing  offspring. 
Darwin’s  contention  is  that  there  will  be  a strong 
tendency  for  those  individuals  which  show  slight 
modifications  in  the  direction  of  a better  adaptation 
to  their  environment  to  survive  at  the  expense  of 
those  of  their  brethren  which  do  not  exhibit  similar 
modifications.  This  is  the  principle  called  natural 
selection  by  Darwin,  and  by  Herbert  Spencer  the 
survival  of  the  fittest.  Let  us  quote  Darwin’s  own 
summary  of  the  process  : 

‘ If  under  changing  conditions  of  life  organic  beings 


48  THE  THEORY  OF  NATURAL  SELECTION 


present  individual  differences  in  almost  every  part  of 
their  structure,  and  this  cannot  be  disputed ; if  there 
be,  owing  to  their  geometrical  rate  of  increase,  a severe 
struggle  for  life  at  some  age,  season,  or  year,  and  this 
certainly  cannot  be  disputed  ; then,  considering  the 
infinite  complexity  of  the  relations  of  all  organic 
beings  to  each  other  and  to  their  conditions  of  life, 
causing  an  infinite  diversity  in  structure,  constitu- 
tion, and  habits,  to  be  advantageous  to  them,  it  would 
be  a most  extraordinary  fact  if  no  variations  had  ever 
occurred  useful  to  each  being’s  own  welfare,  in  the  same 
manner  as  so  many  variations  have  occurred  useful  to 
man.  But  if  variations  useful  to  any  organic  being  ever 
do  occur,  assuredly  individuals  thus  characterized  will 
have  the  best  chance  of  being  preserved  in  the  struggle 
for  life  ; and  from  the  strong  principle  of  inheritance, 
these  will  tend  to  produce  offspring  similarly  charac- 
terized. This  principle  of  preservation,  or  the  survival 
of  the  fittest,  I have  called  Natural  Selection.  It  leads 
to  the  improvement  of  each  creature  in  relation  to  its 
organic  and  inorganic  conditions  of  life,  and,  conse- 
quently, in  most  cases,  to  what  must  be  regarded  as  an 
advance  in  organisation.  Nevertheless,  low  and 
simple  forms  will  long  endure,  if  well  fitted  for  their 
simple  conditions  of  life.’* 

We  have  here  a very  definite  and  concise  statement 
of  the  way  in  which  Darwin  believed  the  principle  of 
natural  selection  to  take  effect  in  the  production  of  new 
kinds  of  organisms.  It  will  be  our  business  in  this  and 
in  succeeding  chapters  to  show  how  far  the  modern 
* ‘ Origin  of  Species,’  sixth  edition,  p,  96. 


NATURAL  SELECTION 


49 


study  of  the  nature  of  individual  differences  and  of 
other  kinds  of  variations,  as  well  as  of  the  manner  of 
operation  of  ‘ the  strong  principle  of  inheritance,’  has 
confirmed  this  view  as  to  the  method  of  origin  of 
species,  or  has  led  to  the  introduction  of  modifications. 

Let  it  be  remembered  that  this  suggestion  of  a 
natural  means  of  modification  had,  within  a few  years, 
the  effect  of  convincing  practically  the  whole  thinking 
world  of  the  truth  of  the  theory  of  organic  evolution — 
an  effect  which  all  the  other  arguments  recited  in  the 
last  chapter  were  quite  unable  to  produce,  so  strong 
was  the  then  existing  prejudice  in  favour  of  the  doc- 
trine of  special  creation. 

The  truth  of  the  general  principle  of  the  survival  of 
the  fittest  is  quite  untouched  by  recent  criticism  ; 
but  a great  deal  of  argument  has  been  expended  over 
the  questions : (i)  how  much  fitness  is  sufficient  to 
lead  to  survival,  and  (2)  whether  very  small  advan- 
tages in  the  way  of  fitness,  even  if  they  lead  to  the  sur- 
vival of  the  individuals  which  exhibit  them,  will  be 
followed  to  an  indefinite  extent  in  succeeding  genera- 
tions by  further  improvements  in  the  same  direction. 
We  shall  find  that  a good  deal  of  evidence  has  accu- 
mulated tending  to  show  that  the  second  of  these 
questions  must  be  answered  in  the  negative,  although 
the  point  is  not  yet  settled  to  the  satisfaction  of 
everyone.  The  remainder  of  the  present  chapter  will 
be  occupied  in  discussing  some  of  the  arguments  which 
bear  upon  this  question. 

An  important  factor  which  must  probably  be  added 
to  the  factors  of  variation,  selection,  and  heredity,  in 

4 


50  THE  THEORY  OF  NATURAL  SELECTION 


order  to  complete  the  Darwinian  account  of  the  origin 
of  species,  is  that  of  segregation  or  isolation.  If  the 
selection  of  minute  fortuitous  variations  in  different 
directions  is  capable  of  breaking  up  a species  into  a 
number  of  new  species,  it  seems  clear  that  this  can 
only  happen  when  the  members  of  the  different  branches 
are  prevented  from  interbreeding  ; since  otherwise  the 
effect  of  selection  would  be  counteracted  by  the 
mingling  or  blending  of  characters  which  may  be  sup- 
posed to  result  from  free  intercrossing.  Further,  many 
zoologists,  and  more  especially  the  systematists  among 
them,  believe  that  isolation  in  itself  has  a most  im- 
portant function  in  modifying  species.  This  isolation 
may  be  either  geographical,  as  when  distance  or  some 
physical  barrier  separates  different  members  of  the 
same  species  ; or  it  may  be  physiological,  as  when 
structural  or  temperamental  differences,  or  mutual 
distaste,  prevent  the  mating  of  certain  individuals. 

The  researches  of  Gulick  upon  the  species  of  snails 
found  in  the  Haiwaian  Islands  showed  that  the  differ- 
ences between  the  species  correspond  in  amount  with 
their  degree  of  separation  in  space — i.e.,  with  their 
isolation.  The  characters  which  separate  these  species 
could  not  be  shown  to  have  any  relation  to  differences 
in  the  environment,  since  adjoining  valleys,  which 
differed  considerably  in  vegetation  and  rainfall,  pos- 
sessed closely  related  species  ; whilst  in  valleys  further 
apart,  but  more  similar  in  the  environment  offered 
to  the  snails,  the  characteristic  species  showed  much 
greater  differences.  The  case  of  the  Haiwaian  snails, 
therefore,  appears  to  afford  an  exception  to  the  prin 
ciple  next  to  be  described. 


NATURAL  SELECTION 


51 


The  fact  that  organic  beings  on  the  whole  are,  as  a 
general  rule,  very  closely  fitted  for  the  conditions  in 
which  they  have  to  pass  their  lives  is  clearly  shown 
by  the  study  of  adaptations.  This  is  a subject  which 
those  followers  of  Darwin  who  believe  in  the  all- 
sufEiciency  of  natural  selection  have  brought  into  con- 
siderable prominence.  For  a full  account  of  many 
supposed  beautiful  adaptations,  from  the  point  of  view 
of  the  most  prominent  member  of  the  school  in  ques- 
tion, reference  may  be  made  to  Weismann’s  book, 

‘ The  Evolution  Theory.’ 

On  the  theory  of  natural  selection  in  its  extreme 
form,  all  the  parts  of  an  animal  or  plant — or,  at  any  rate, 
all  the  points  in  which  one  species  differs  from  another 
nearly  related  species — are  supposed  to  have  arisen  on 
account  of  their  usefulness  to  the  creatures  possessing 
them.  Every  detail  of  structure  is  thus  regarded  as 
being  more  or  less  closely  adapted  to  the  circum- 
stances which  attend  the  life  of  the  animal  or  plant  in 
question.  This  adaptation  is  never,  indeed,  regarded 
as  perfect,  because  natural  selection  is  always  in 
progress,  and  its  work  is  never  absolutely  done ; 
but  the  point  is  that  the  features  of  every  part  are 
aimed  at  some  useful  purpose  ; or,  if  they  are  not, 
then  they  have  been  useful  in  former  times  and  under 
different  circumstances,  and  are  now  undergoing  a 
process  of  gradual  removal,  because  the  individuals  in 
which  the  useless  structure  is  least  developed  will  now 
have  the  best  chance  of  surviving.  That  the  form  and 
structure  of  an  animal  or  plant  is  in  general  closely 
fitted  to  its  environment  is  of  course  true  ; otherwise 

4—2 


52  THE  THEORY  OF  NATURAL  SELECTION 


the  creature  would  very  soon  cease  to  cumber  the  earth. 
But  the  student  of  adaptation  goes  into  details,  and 
endeavours  to  find  a use  for  every  minute  point  of 
structure,  on  the  assumption,  which  we  shall  presently 
see  to  be  open  to  criticism,  that  but  for  their  useful- 
ness these  details  would  not  exist.  We  may  proceed 
to  glance  at  one  or  two  examples  of  the  kind  of  thing 
which  is  meant  when  it  is  said  that  an  animal  or  plant 
exhibits  very  marked  adaptative  features. 

The  order  Cetacea  belongs  to  the  class  of  mammals 
of  which  the  more  typical  members  are  land  animals 
possessing  four  legs,  and  having  their  bodies  covered 
with  hair.  The  true  whalebone  whales,  a sub-order 
which  includes  the  Greenland  whale,  are  in  many  ways 
the  most  specialized  members  of  the  group. 

The  Greenland  whale*  has  a spindle-shaped  body 
like  that  of  a fish,  and  its  fore  limbs  are  modified  into 
flippers  resembling  the  pectoral  fins  of  fishes.  The 
hind  legs  are  only  represented  by  a few  rudimentary 
bones,  which  are  completely  hidden  within  the  body 
wall,  and  the  function  of  propulsion,  which  is  performed 
by  the  hind  legs  in  such  less  completely  aquatic  animals 
as  seals,  is  here  taken  over  by  a great  tail-fin  which 
resembles  that  of  a fish,  except  that  it  is  placed  hori- 
zontally. Hair  is  absent,  but  under  the  skin  a thick 
layer  of  blubber  is  developed,  which  prevents  a too 
rapid  loss  of  heat,  and  at  the  same  time  adjusts  the 
specific  gravity  of  the  body  to  that  of  the  surrounding 
water.  External  ears  are  entirdy  wanting,  and  the 

* Weismann,  ‘ The  Evolution  Theory,’  English  edition, 

ii.  313. 


ADAPTATION 


53 


waves  of  sound  are  apparently  transmitted  to  the 
drum  of  the  ear  directly  through  the  bones  of  the  head. 
The  external  openings  of  the  nostrils  are  placed  quite 
on  the  upper  surface  of  the  head,  so  that  the  animal 
can  breathe  whilst  almost  completely  submerged ; and 
the  larynx  is  so  modified  that  the  function  of  swallowing 
does  not  interfere  with  that  of  breathing.  Perhaps  the 
most  remarkable  feature  of  all  is  the  enormous  develop- 
ment of  the  head,  and  especially  of  the  mouth.  The 
huge  jaws,  in  combination  with  the  extraordinary  plates 
of  whalebone  which  fringe  the  edges  of  the  mouth  and 
act  as  a sieve,  enable  the  animal  to  get  its  nutriment 
from  the  minute  free-swimming  creatures  with  which 
the  surface  waters  of  the  ocean  abound.  Associated 
with  this  special  method  of  feeding  is  the  fact  that 
teeth  are  only  to  be  recognised  in  the  embryo,  and 
afterwards  entirely  disappear. 

The  whales  differ  in  all  these  points  from  any  other 
mammals,  and  failing  almost  any  of  these  differences, 
would  not  be  able  to  live  in  the  special  conditions  in 
which  they  find  themselves.  It  must  therefore  be 
admitted  that  we  have  here  a case  of  very  close  adapta- 
tion of  an  animal  to  its  natural  surroundings,  and  one 
which  extends  to  almost  every  detail  of  its  structure. 
Darwin  himself,  moreover,  has  been  at  special  pains  to 
show  how  some  of  the  most  remarkable  of  these 
structural  adaptations  may  possibly  have  arisen 
through  natural  selection. 

One  of  the  most  remarkable  cases  of  mutual  adapta- 
tion, in  which  an  animal  and  a plant  are  associated 
together,  is  shown  by  the  method  of  fertilization 


54  THE  THEORY  OF  NATURAL  SELECTION 


observed  to  take  place  in  the  flowers  of  the  Yucca 
plant  of  the  Southern  United  States.  The  act  of 
pollination  is  performed  by  a moth — Pronub  a — ^which 
possesses  special  organs  particularly  adapted  for  this 
purpose,  in  the  shape  of  peculiar  maxillary  tentacles 
which  are  found  in  no  other  kind  of, moth.  The  female 
has  also  a long  ovipositor  with  which  she  can  pierce 
the  tissues  of  the  ovary  of  the  plant,  and  so  lay  her 
eggs  within  it.  With  the  aid  of  her  peculiar  tentacles  the 
female  moth  collects  from  several  flowers  a ball  of  pollen 
of  considerable  size,  which  she  kneads  into  a firm  pellet. 
She  then  carries  this  to  a different  flower^  and  after 
depositing  a few  eggs  in  the  ovary  she  climbs  to  the 
top  of  the  style  and  presses  the  ball  of  pollen  into  the 
stigma.  Thus  the  ovules  of  the  flower  are  fertilized^ 
and  whilst  some  are  eaten  by  the  larvae  of  the  moth, 
others  develop  into  seeds  and  reproduce  the  plant. 

The  foregoing  are  perhaps  two  of  the  most  remarkable 
cases  known  of  animals  having  peculiar  habits,  and 
possessing  at  the  same  time  special  organs  which  render 
them  well  fitted  for  these  habits  and  no  others ; but 
many  other  cases  of  scarcely  less  wonderful  adapta- 
tions have  been  pointed  out. 

Darwin  himself  indicated  the  direction  in  which  the 
study  of  adaptation  was  to  proceed,  and  his  books  on 
‘Insectivorous  Plants’  and  on  the  ‘Fertilization  of 
Orchids  ’ afford  us  a delightful  insight  into  a number  of 
adaptive  contrivances  which  are  to  be  seen  in  plants. 
Another  very  interesting  series  of  adaptive  characters 
are  those  which  have  been  gathered  together  under 
the  heads  of  Protective  Resemblance  and  Mimicry,  and 


MIMICRY 


55 


these  have  a special  interest  for  us,  because  they  illus- 
trate the  way  in  which  the  zeal  of  the  seeker  after  adap- 
tive contrivances  may  run  away  with  him  if  not  kept 
well  in  hand.  For  there  is  scarcely  any  limit  to  the  num- 
ber of  problematical  cases  which  have  been  described 
as  adaptive  resemblances,  and  so  explained  as  having 
arisen  through  natural  selection,  whilst  the  evidence 
in  favour  of  such  a supposition  is  in  many  cases  highly 
questionable.  On  the  other  hand,  in  a number  of 
well-marked  instances,  the  theory  of  mimicry  certainly 
seems  to  afford  an  adequate  explanation  of  the  way 
in  which  many  curious  characters  and  structures  may 
possibly  have  come  into  existence. 

The  families  of  the  Mantidcs  and  Phasmidce,  includ- 
ing the  walking-stick  and  leaf  insects,  afford  many 
examples  of  animals  which  both  in  their  colour  and 
configuration  show  a very  close  resemblance  to  sur- 
rounding inanimate  objects.  This  resemblance  must 
have  the  effect  of  concealing  them  from  their  enemies, 
and  more  particularly  from  their  prey,  as,  indeed,  a 
study  of  their  habits  indicates  quite  clearly. 

Phyllopteryx,  an  Australian  fish  allied  to  the  well- 
known  sea-horse  {Hij>pocampus),  is  provided  with  a 
number  of  irregular  appendages  of  ragged  skin 
resembling  the  seaweed  amongst  which  this  animal 
is  found.  In  this  way  the  characteristic  symmetrical 
appearance  of  a live  animal  is  got  rid  of,  and  the 
creature  is  rendered  extremely  difficult  of  observation. 
Here,  again,  the  concealment  afforded  is  probably 
useful  in  leading  to  the  deception  of  the  smaller 
organisms  upon  which  the  creature  feeds. 


56  THE  THEORY  OF  NATURAL  SELECTION 


Examples  of  this  kind  in  which  the  shape  of  an 
animal  leads  to  its  concealment  are  less  numerous 
than  those  in  which  protection  is  afforded  by  an  obscure 
pattern  or  inconspicuous  colour.  Indeed,  some  re- 
semblance between  the  colour  of  an  animal  and  its 
surroundings  is  to  be  traced  in  the  majority  of  the 
members  of  many  groups.  Familiar  examples  are 
afforded  by  the  white  colour  of  animals  which  live  in 
snow,  the  tawny  grey  colour  of  most  desert  species, 
the  green  of  grass-frequenting  animals,  and  so  on.  It 
is  perhaps  not  quite  certain  that  in  some  of  these  cases 
the  peculiar  colour  is  not  evoked  by  the  direct  action 
of  some  cause  which  affects  different  species  in  the  same 
way ; but  such  a cause  awaits  discovery,  and  in  the 
meantime  natural  selection  has  certainly  a strong 
claim  to  be  regarded  as  the  proper  explanation. 

A more  strict  use  of  the  term  mimicry,  however,  is 
to  restrict  it  to  cases  where  one  species  apes  the  colour 
pattern  or  other  external  character  proper  to  another 
species  which  inhabits  the  same  region ; and  the  idea 
of  mimicry  has  been  put  forward  as  especially  appro- 
priate in  cases  where  the  mimicked  species  is  common, 
and  can  be  thought  to  possess  some  special  means  of 
protection.  Numerous  supposed  examples  of  this 
phenomenon  have  been  described  among  insects,  espe- 
cially in  the  case  of  various  butterflies  from  Africa, 
Malaya,  and  South  America.  It  would  be  beyond  the 
scope  of  this  work  to  do  more  than  call  attention  to  the 
fascinating  subject,  the  literature  of  which  includes  a 
large  number  of  papers  to  be  found  in  the  Proceedings 
of  the  Linnaean  Society  and  elsewhere.  For  a general 


MIMICRY 


57 


account  of  this  work,  the  reader  is  referred  to  Professor 
Poulton’s  recent  ‘ Essays  on  Evolution,’  and  to  Pro- 
fessor R.  C.  Punnett’s  ‘ Mimicry  in  Butterflies.’ 

Manystudents  of  evolution  in  its  more  recent  develop- 
ments are  disposed  to  attach  greater  importance  than 
does  Professor  Poulton  to  the  difficulties  which  beset 
the  theory  of  mimicry,  in  so  far  as  the  theory  consists 
in  explaining  these  resemblances  by  natural  selection 
accumulating  minute  variations  in  the  proper  direction. 
Indeed,  the  power  of  this  evolutionary  factor  seems 
here  to  be  stretched  to  its  utmost  limits  of  tension. 
The  independent  evolution  of  a similar  external  appear- 
ance has  certainly  taken  place  in  some  cases  in  which 
any  suggestion  of  mimicry  is  excluded,  and  there  is 
nothing  to  prove  that  colour-patterns  of  the  same  type 
may  not  have  arisen  from  the  same  causes  in  widely 
different  groups.  In  cases  where  the  environment  to 
which  the  different  forms  were  exposed  was  similar — 
as  would  be  the  case  especially  in  any  single  locality — 
such  a process  of  parallel  evolution  might  be  thought 
to  be  all  the  more  likely. 

It  is  not  to  be  supposed  that  we  intend  for  a moment 
to  impugn  the  reality  of  these  marvellous  resem- 
blances. The  smallest  acquaintance  with  the  facts 
must  show  the  absurdity  of  any  such  suggestion,  just 
as  the  multiplicity  of  the  cases  described  renders  any 
suggestion  of  coincidence  ridiculous.  It  is  only  the 
current  explanation  of  these  resemblances  to  which  we 
take  exception,  for  the  brain  reels  before  the  task  of 
picturing  the  gradual  building  up  of  such  a resemblance 
by  the  successive  additions  of  small  differences,  each 
one  useful  to  the  possessor  of  it. 


58  THE  THEORY  OF  NATURAL  SELECTION 


Resemblances  can  only  be  properly  explained  as 
representing  cases  of  mimicry  when  both  the  species 
concerned — the  mimic  and  the  mimicked — inhabit  the 
same  locality ; but  plenty  of  cases  of  matching  between 
the  colour  patterns  of  insects  which  live  in  quite 
different  parts  of  the  world  could  also  be  pointed  out. 
Let  us  take  a concrete  example.  Everyone  is  familiar 
with  the  flower-frequenting  flies  {Syrphidce)yfh.ich.  are  to 
be  seen  hovering  about  plants  in  sunny  weather.  These 
insects  closely  mimic  the  appearance  of  various  small 
bees  and  wasps,  the  habits  of  which  are  similar.  Here, 
then,  is  surely  a case  where  the  deceptive  resemblance 
to  an  animal  well  armed  in  its  sting  must  cause  pro- 
spective enemies  to  let  these  flies  alone.  In  Southern 
Japan,  as  Dr.  Andrese  pointed  out  to  me,  flies  of  this 
kind  are  surprisingly  numerous,  and  their  resemblance 
to  bees  particularly  noticeable.  So  abundant  are  they 
that,  from  the  point  of  view  of  the  flowers  which  they 
visit,  these  flies  doubtless  provide  an  efflcient  substitute 
for  the  bees  of  other  countries,  which  are  here  con- 
spicuous by  their  absence.  But  if  real  stinging  insects 
are  wanting,  or  even  very  scarce,  the  supposed  enemies 
of  the  flies  can  have  no  experience  of  the  ill-effects 
produced  by  catching  them.  How,  then,  can  these 
flies  benefit  from  their  resemblance  to  bees  ? 

This  kind  of  thing  must  make  us  somewhat  sus- 
picious of  supposed  cases  of  mimicry  even  between 
species  possessing  the  same  range. 

When  the  ideas  of  mimicry  and  protective  resem- 
blance are  carried  into  the  vegetable  kingdom,  as  they 
have  been  by  some  writers,  absurdities  are  soon  found 


PARALLEL  EVOLUTION 


59 


to  arise.  Thus  it  has  been  suggested  that  the  leaves 
of  dead  nettles  resemble  those  of  the  common  nettle 
for  the  sake  of  the  protection  so  afforded,  and  that  the 
mottled  stems  of  certain  tropical  herbaceous  plants 
gain  a similar  immunity  on  account  of  their  resemblance 
to  snakes. 

In  plants  a great  number  of  fanciful  resemblances 
between  different  species  can  be  detected,  and  some 
between  plants  and  animals,  very  few  of  which  can  be 
supposed  to  be  of  any  possible  utility  to  the  species 
which  exhibit  them.  They  must  be  regarded  as  cases 
of  parallel  evolution,  the  causes  of  which  are  quite 
unknown.  Such  resemblances  as  that  between  the 
shoots  of  Casuarina  indica  and  those  of  the  common 
horse-tail,  between  Saxifraga  hypnoides  and  certain 
mosses,  between  the  horse-  and  Spanish-chestnut,  be- 
tween the  seed  of  a pine  and  the  fruit  of  an  ash-tree, 
are  so  frequent  in  the  vegetable  kingdom  as  to  be  the 
delight  of  malicious  examiners  in  elementary  botany. 
It  is  impossible  to  believe  that  in  such  cases  the 
resemblance  is  in  itself  of  any  value  to  either  species, 
and  few  people  wiU  be  found  to  maintain  that  the 
likeness  of  a bee-  or  spider-orchis  to  an  insect  is  of  any 
utility  to  either  animal  or  plant. 

But  if  resemblances  can  arise  which  are  useless,  and 
which,  consequently,  cannot  be  explained  through 
natural  selection,  it  becomes  uncertain  whether  this 
principle  can  hold  good  as  the  true  description  of  the 
origin  of  any  sort  of  resemblance.  On  the  other  hand, 
resemblances  which  are  useful  will  tend  to  survive 
through  natural  selection  in  whatever  way  they  may 


6o  THE  THEORY  OF  NATURAL  SELECTION 


have  arisen.  This  last  consideration  will  account  for 
the  frequency  with  which  apparently  adaptive  like- 
nesses are  to  be  found  in  nature,  even  if  we  suppose 
that  their  origin  was  ‘ accidental,’  or  simply  due  to 
the  operation  of  similar  external  causes.  The  same 
criticism  applies  to  all  cases  of  adaptation  of  whatever 
kind,  so  far  as  concerns  their  supposed  method  of  origin 
by  the  action  of  natural  selection  upon  individual 
differences. 

Perhaps  a still  more  serious  criticism  of  the  methods 
of  those  who  spend  their  time  in  seeking  out  or  devising 
cases  of  adaptation  has  been  made  by  Bateson,  who 
points  out  the  logical  difficulty  that  we  can  never 
make  any  quantitative  estimate  of  the  amount  of 
benefit  or  the  reverse  which  any  particular  structure 
may  afford  to  its  possessor.  It  is  easy  enough  to 
imagine  particular  circumstances  in  which  an  organ 
developed  in  a particular  way  may  be  of  undoubted 
service,  but  whether  the  net  amount  of  such  service 
throughout  the  life  of  the  creature  considered  is 
greater  or  less  than  the  strain  upon  its  resources  caused 
by  the  development  of  such  an  organ  is  quite  beyond 
our  powers  of  determination. 

‘ The  students  of  adaptation  forget  that  even  on  the 
strictest  application  of  the  theory  of  selection  it  is 
unnecessary  to  suppose  that  every  part  an  animal  has, 
and  everything  which  it  does,  is  useful  and  for  its  good. 
We,  animals,  live  not  only  by  virtue  of,  but  also  in 
spite  of  what  we  are.  It  is  obvious  from  inspection 
that  any  instinct  or  organ  may  be  of  use  ; the  real 
question  we  have  to  consider  is  how  much  use  it  is. 


REGENERATION 


6i 


To  know  that  the  presence  of  a certain  organ  may  lead 
to  the  preservation  of  a race  is  useless  if  we  cannot  tell 
how  much  preservation  it  can  effect,  how  many  indi- 
viduals it  can  save  that  would  otherwise  be  lost ; unless 
we  know  also  the  degree  to  which  its  presence  is 
harmful ; unless,  in  fact,  we  know  how  its  presence 
affects  the  profit  and  loss  account  of  the  organism.’* 

A great  many  other  criticisms  and  objections  have 
been  brought  at  various  times  against  the  theory  of 
the  origin  of  adaptations  by  the  action  of  natural 
selection,  and  many  of  these  were  considered  and 
replied  to  by  Darwin  in  the  later  editions  of  the  ‘ Origin 
of  Species.’  We  shall  only  consider  here  a few  which 
have  been  put  forward  more  or  less  recently.  Before 
doing  so  it  will  be  well  to  point  out  once  more  that  no 
one  questions  the  validity  of  natural  selection  as  a means 
of  exterminating  types  which  are  unfitted  for  their 
environment — there  is  clearly  a tendency  for  the 
fittest  types  to  survive  once  they  have  come  into 
existence.  Nor  can  there  be  any  doubt  that  species 
in  general  are  well  adapted  to  the  conditions  which 
their  environments  present.  But  when  this  is  admitted 
it  does  not  necessarily  follow  that  natural  selection, 
directing  accumulation  of  minute  differences,  has 
been  the  method  by  which  these  adapted  forms  have 
originated.  ' 

The  power  of  regenerating  a lost  part  must  clearly 
often  be  of  service  to  the  creatures  which  possess  it. 
Such  a power  may  in  many  cases  be  considered  to  be 
a weU-marked  adaptation.  But,  as  Morgan  has  well 
• W.  Bateson,  ‘ Materials  for  the  Study  of  Variation,’  p.  12, 


62  THE  THEORY  OF  NATURAL  SELECTION 


pointed  out,  there  are  insuperable  difficulties  in  the 
way  of  adopting  the  belief  that  such  a power  can  have 
been  acquired  through  the  action  of  natural  selection. 
Many  crustaceans,  when  they  have  lost  a claw  or  limb, 
proceed  straightway  to  grow  a new  one.  To  account 
for  this  power  by  natural  selection  we  should  have  to 
suppose,  firstly,  that  every  stage  in  the  growth  of  a 
partly  regenerated  claw,  even  its  first  small  rudiment, 
was  useful  to  the  animal ; and,  secondly,  that  there 
was  so  much  competition  between  lobsters  which  had 
lost  their  claws,  that  those  which  could  regenerate 
them  a little  better  would  survive  rather  than  the 
others.  The  first  of  these  suppositions  as  to  the  utility 
of  a partly  regenerated  claw  is  in  the  highest  degree 
improbable ; but  against  the  second  there  is  an 
entirely  fatal  argument,  since,  if  the  lobsters  which 
regenerated  badly  were  exterminated  owing  to  com- 
petition with  those  which  had  better  powers  of  re- 
generation, much  more  would  all  the  injured  lobsters 
be  exterminated  in  competition  with  those  which  had 
escaped  injury. 

The  theory  of  sexual  selection  constitutes  an  im- 
portant branch  of  the  Darwinian  account  of  the  origin 
of  specific  structures.  We  are  here  concerned  with 
this  hypothesis  only  in  so  far  as  it  leads  to  a criticism 
of  the  efficacy  of  natural  selection  from  another  point 
of  view . By  the  theory  of  sexual  selection  Darwin  at- 
tempted to  explain  the  origin  of  two  sorts  of  characters 
in  particular,  one  or  other  of  which  frequently  appears 
in  the  male  sex  only  of  many  of  the  higher  animals. 


SEXUAL  SELECTION 


63 


In  the  first  place,  we  have  to  notice  the  presence  of 
special  weapons,  such  as  horns  or  tusks,  developed 
exclusively  or  to  a special  extent  in  the  males  of 
those  species  in  which  it  is  the  habit  of  the  members 
of  this  sex  to  strive  together  for  the  possession  of  the 
females.  In  such  cases  the  stronger  and  better-armed 
males  are  supposed  to  survive,  and  to  leave  a greater 
number  of  offspring  than  their  weaker  rivals,  so  that 
this  form  of  competition  is  regarded  as  acting  in 
quite  a similar  way  to  natural  selection. 

In  a second  set  of  cases,  of  which  many  remarkable 
instances  are  to  be  seen  among  birds,  the  males  are 
found  to  exhibit  brilliant  and  varied  colours,  or  to 
possess  special  decorations  in  the  form  of  plumes  or 
other  appendages,  or  to  be  gifted  with  the  power  of 
song.  It  is  to  cases  such  as  these  that  the  term  sexual 
selection  more  properly  applies,  since  the  females  are 
supposed  to  bestow  their  favours  upon  the  most 
beautiful  males,  and  to  reject  the  advances  of  those 
among  their  suitors  which  are  less  lavishly  provided 
with  ornament. 

In  these  cases,  where  the  development  of  brilliant 
colours  or  other  ornamental  arrangements  is  believed 
to  have  taken  place  owing  to  the  choice  of  the  females 
— particularly  in  such  a case  as  is  represented  by  the 
peacock’s  tail  or  the  wings  of  the  Argus  pheasant — 
the  supposed  change  must  have  come  about  in  direct 
opposition  to  the  action  of  natural  selection,  since  the 
latter  would  favour  a production  of  colours  resembling 
those  of  the  natural  environment  for  the  sake  of  con- 
cealment, and  would  hinder  the  formation  of  such 


64  the  theory  of  NATURAL  SELECTION 


exaggerated  appendages  on  account  of  the  loss  of 
activity  which  they  must  entail.  We  are,  therefore^ 
obliged  to  conclude  that  natural  selection  is  much  less 
rigorous  in  its  action  than  some  people  have  supposed,; 
for  if  this  principle  is  inadequate  to  prevent  such  an 
exuberance  of  form  and  colour  in  these  particular 
instances,  its  action  becomes  open  to  question  in 
other  cases  as  well. 

Similarly,  Morgan  finds  a difficulty  in  understanding 
why  natural  selection  has  not  led  to  the  extermination 
of  species  which  are  handicapped  by  the  existence  of 
internecine  strife  between  the  males,  in  favour  of  other 
species  which  faced  the  battle  of  life  with  united 
strength.  But  in  this  argument  it  seems  to  be  for- 
gotten that  examples  of  the  kind  of  strife  in  question 
are  most  frequent  among  herbivorous  animals,  where 
the  struggle  for  existence  must  be  chiefly  determined 
by  the  quantity  of  vegetable  food  which  the  individuals 
can  obtain,  so  that  the  loss  of  the  weaker  males  may 
not  be  a disadvantage.  Moreover,  Darwin’s  conclusion 
that  natural  selection  is  most  rigorous  between  members 
of  the  same  species  is  left  out  of  account. 

The  preceding  arguments  seem  to  show  that  in  par- 
ticular cases  certain  structures  and  phenomena  asso- 
ciated with  species  cannot  be  explained  as  having 
arisen  through  the  unaided  action  of  natural  selection. 
When  weighed  against  the  great  mass  of  evidence 
which  Darwin  accumulated  in  support  of  his  theory, 
these  few  considerations  cannot  be  said  to  be  in  any 
way  fatal  to  the  belief  that  natural  selection  of  minute 
differences  has  played  an  important  part  in  the  origin 


INHERITANCE  OF  ACQUIRED  CHARACTERS  65 


of  species.  Still,  they  add  in  some  measure  to  the 
weight  of  recent  evidence  which  points  to  the  con- 
clusion that  many  specific  structures  have  had  a 
different  method  of  origin.  We  have  already  pointed 
out  that  there  are  two  alternative  methods,  each  of 
which  has  its  adherents.  Before  passing  to  a con- 
sideration of  the  now  prevalent  view  of  mutationt 
something  still  remains  to  be  said  with  regard  to  the 
remaining  theory — the  theory  of  Lamarck. 

Darwin  himself,  as  we  have  seen,  admitted  the 
minor  importance  of  the  inheritance  of  acquired 
^iharacters,  as  well  as  that  of  the  phenomenon  of 
sporting,  regarding  both  these  processes  as  causes  of 
the  origin  of  new  species  subsidiary  to  the  action  of 
natural  selection  upon  individual  differences,  whilst  he 
looked  upon  the  latter  as  the  main  process  in  organic 
evolution. 

Later  writers,  however,  have  asserted  that  natural 
selection  is  the  sole  cause  of  the  origin  of  species,  and 
in  particular  they  have  denied  any  effect  to  the  in- 
heritance of  acquired  characters — the  Lamarckian 
factor — asserting  that  there  is  not,  and  cannot  be, 
any  such  inheritance.  Among  the  most  distinguished 
opponents  of  the  theory  of  use-inheritance  were  A.  R. 
Wallace,  the  co-discoverer  of  natural  selection  ; and 
Professor  Weismann,  who  has  argued  the  case  with 
particular  ability.  Much  the  most  able  defender  of 
the  principle  of  use-inheritance  was  Herbert  Spenceri 
who  was  one  of  the  few  who  had  thoroughly  convinced 
themselves  of  the  truth  of  the  theory  of  evolution 
y^ars  before  the  ‘ Origin  of  Species  ’ made  its  appear- 

5 


66  THE  THEORY  OF  NATURAL  SELECTION 


ance.  Since  all  arguments  in  favour  of  the  evolution 
of  species  were  incomplete  unless  some  means  by 
which  such  an  evolution  could  take  place  had  been 
suggested,  Spencer  adopted  the  Lamarckian  theory  of 
modification,  and  to  this  he  always  firmly  adheredi 
though  admitting  the  validity  of  natural  selection  as 
an  additional  factor  in  the  process.  Some  of  Spencer’s 
arguments  in  favour  of  a belief  in  the  inheritance  of 
acquired  characters  are  well  worth  repetition,  since 
they  have  never  been  altogether  refuted. 

Herbert  Spencer’s  argument  consisted  mainly  in  the 
enumeration  of  structures  the  origin  of  which  cannot 
be  explained  by  natural  selection.  On  the  other  hand, 
the  inheritance  of  acquired  characters,  if  this  form  of 
inheritance  could  be  proved  to  have  a real  existence 
— as  Spencer  believed  it  could — ^was  shown  to  be  a 
perfectly  adequate  explanation  of  the  origin  of  the 
structures  in  question.  In  1893,  when  Spencer  up- 
held his  opinion  for  the  last  time,  Bateson  had  not  yet 
pointed  out  that  the  facts  of  definite  and  discontinuous 
variation  afford  an  alternative  way  out  of  some  of 
these  difficulties.  In  the  absence,  therefore,  of  any 
other  effective  cause,  the  result  of  the  argument 
pointed  strongly  to  the  conclusion  that  the  inheritance 
of  acquired  characters  must  be  a reality. 

The  first  of  Spencer’s  arguments  was  based  upon 
the  different  powers  of  tactual  discrimination  which 
are  to  be  found  in  different  parts  of  the  human  body- 
The  degree  of  this  sensitiveness  may  be  estimated  by 
the  use  of  a pair  of  compassesi  the  points  of  which  can 
be  set  at  different  distances  apart.  It  is  then  found 


DISCRIMINATION  BY  TOUCH 


67 


that  with  the  tip  of  the  forefinger  the  points  can  be 
distinctly  recognised  as  two  when  they  are  separated 
by  no  more  than  inch.  When  applied  to  the  middle 
of  the  back,  on  the  other  hand,  the  points  must  be 
opened  to  a distance  of  2J  inches  before  the  sensation 
of  a single  touch  becomes  resolved  into  two  distinct 
sensations. 

The  distribution  of  this  power  of  discrimination  over 
the  surface  of  the  body  is  approximately  as  follows  ] 


Tip  of  tongue  

Tip  of  finger . . 

Inner  surface  of  second  joint  of 
finger  . . 

Tip  of  nose  . . 

Cheek,  palm  of  hand,  and  end  of 
great-toe 

Forehead 

Back  of  hand,  crown  of  head 

Breast 

Middle  of  back,  middle  of  thigh, 
middle  of  forearm 


inch. 

1 

Tir 

i a 
i 


,1 


xi 


Now,  it  is  out  of  the  question  to  suppose  that  natural 
selection  can  account  for  aU  these  differences.  An 
increased  sensitiveness  of  the  tips  of  the  fingers  might, 
indeed,  be  of  so  much  use  as  to  give  the  individual 
possessing  it  a definitely  increased  chance  of  survival. 
But  it  is  hard  to  believe  that  it  can  be  important  for 
a man  to  have  the  tip  of  his  tongue  twice  as  sensitive 
as  the  tips  of  his  fingers.  And  why  should  the  tip  of 
the  nose  be  more  sensitive  than  the  cheek,  or  the 
cheek  than  the  top  of  the  head,  or  the  breast  than 
the  back  ? In  the  last  case  it  might  even  be  suggested 

5—2 


68  THE  THEORY  OF  NATURAL  SELECTION 


that  in  a savage,  since  the  sense  of  touch  is  the  only 
one  with  which  his  back  is  provided,  it  might  be 
useful  for  that  surface  to  have  acquired  a more  deli- 
cate sense  of  touch  than  the  anterior  surface,  which 
is  guarded  by  the  power  of  vision,  as  well  as  being 
more  readily  explored  by  the  sensitive  finger-tips. 
If  such  an  argument  is  regarded  as  far-fetched,  so  in 
an  equal  degree  must  be  any  attempt  to  explain  the 
actually  observed  distribution  through  the  action  of 
natural  selection. 

On  the  other  hand,  Spencer  points  out  that  the 
series  of  parts  enumerated  in  the  above  table  stands 
in  almost  exactly  the  order  of  the  frequency  with 
which  the  members  composing  it  are  actually  exposed 
to  tactual  experience. 

The  tongue  is  perpetually  in  contact  with  the  minute 
unevennesses  afforded  by  the  surfaces  of  the  teeth. 

The  palm  of  the  hand  and  the  lower  joints  of  the 
fingers  are  used  chiefly  in  grasping,  and  not  in  the 
more  minute  manipulations  for  which  the  finger-tips 
are  employed.  And  the  experience  of  the  back  of  the 
hand  in  coming  into  contact  with  various  irregular 
bodies  is  not  to  be  compared  with  that  of  the  palmar 
surface,  yet  it  is  very  much  greater  than  that  of  so 
unexposed  a part  as  the  middle  of  the  forearm. 

For  the  carrying  on  of  his  argument,  Herbert  Spencer 
has  shown  that  increased  use  of  the  power  of  dis- 
criminating small  objects  by  touch  is  accompanied  by 
an  increased  degree  of  sensitiveness  in  individuals. 
Blind  people  use  their  finger-tips  in  this  way  to  a much 
greater  extent  than  those  whose  sight  is  unimpaired. 


CO-ORDINATED  STRUCTURES 


69 


Two  blind  boys  examined  by  Spencer  were  both  found 
to  be  able  to  distinguish  with  the  tips  of  their  fingers 
points  separated  by  only  inch.  And  two  skilled 
compositors  could  both  distinguish  in  this  way  points 
placed  no  more  than  xV  inch  apart,  so  that  a person 
with  a trained  sense  of  touch  acquires  a considerably 
finer  development  of  this  faculty  than  an  ordinary 
individual. 

If,  then,  acquired  characters  of  this  kind  are  in- 
herited, even  to  an  extremely  minute  extent,  such  as 
would  be  scarcely  perceptible  in  a single  generation, 
the  account  of  the  origin  of  the  observed  phenomena 
would  be  complete. 

As  a second  argument,  Herbert  Spencer  points  out 
the  difficulty  of  accounting  for  the  development  of 
co-ordinated  sets  of  structures  by  the  action  of  natural 
selection  upon  separate  minute  variations  of  the  several 
parts  concerned. 

The  enormous  horns  of  the  ancient  Irish  elk,  weigh- 
ing in  some  cases  over  a hundredweight,  required 
specially  strong  neck  muscles,  bones,  and  ligaments, 
and  strong  fore  legs  for  their  support.  But  an  increase 
in  the  strength  of  a single  muscle  following  increased 
weight  of  the  horns  would  be  useless  if  unaccompanied 
by  a corresponding  increase  in  many  other  structures, 
and,  if  useless,  could  not  be  selected.  The  chance  of 
all  the  parts  concerned  varying  simultaneously  in  a 
corresponding  direction  is  very  small  if  these  variations 
are  really  independent,  and  the  chance  of  their  doing 
so  repeatedly  is  in  such  a case  infinitesimal. 

Let  us  take  another  case  of  a similar  nature.  The 


70  THE  THEORY  OF  NATURAL  SELECTION 


hind  legs  of  such  an  animal  as  a cat  are  admirably 
adapted  for  the  purpose  of  making  a spring.  In  order 
to  arrive  at  such  a structure  by  the  modification  of 
limbs  previously  adapted  only  for  running,  changes 
must  occur  in  almost  all  the  bones,  muscles,  and 
ligaments  of  the  limbs,  and  these  changes  must  keep 
pace  with  one  another  so  that  one  part  may  not  grow 
out  of  proportion  with  the  rest.  It  is  quite  impossible 
to  suppose  that  this  can  be  effected  by  the  natural 
selection  of  minute  fortuitous  variations  of  the  various 
parts,  each  occurring  independently.  But  simultane- 
ously with  these  changes  the  fore  legs  have  become 
modified  in  a totally  opposite  direction.  They  have 
become  straight,  firm,  and  pillar-like  for  receiving  the 
weight  of  the  body  in  the  downward  leap.  Compare, 
says  Herbert  Spencer,  the  silence  of  a cat’s  leap  up 
on  to  a table  with  the  thud  made  by  the  fore  legs  as  it 
jumps  down  upon  the  floor. 

Modification  of  the  fore  legs  and  of  the  hind  must 
thus  have  proceeded  in  almost  exactly  opposite  direc- 
tions in  the  two  cases,  and  in  each  a great  number  of 
parts  are  separately  co-ordinated.  For  natural  selec- 
tion to  have  had  any  effect,  all  the  co-ordinated  parts 
of  one  pair  of  legs  must  have  varied  in  one  direction, 
whilst  similar  parts  in  the  other  pair  of  legs  varied 
simultaneously  in  another  direction.  It  is  out  of  the 
question  to  suppose  that  this  could  have  happened 
simply  by  chance. 

‘ What,  then,  is  the  only  defensible  interpretation  ? 
If  such  modifications  of  structure  produced  by  modifi- 
cations of  function  as  we  see  take  place  in  each  indi- 


INHERITANCE  OF  ACQUIRED  CHARACTERS  71 

vidual  are  in  any  measure  transmissible  to  descendants^ 
then  all  these  co-adaptations,  from  the  simplest  up  to 
the  most  complex,  are  accounted  for.  In  some  cases 
this  inheritance  of  acquired  characters  suffices  by  itself 
to  explain  the  facts ; and  in  other  cases  it  suffices  when 
taken  in  combination  with  the  selection  of  favourable 
variations.  An  example  of  the  first  class  is  afforded 
by  the  change  just  considered ; and  an  example  of  the 
second  class  is  furnished  by  the  case,  before  named, 
of  development  in  a deer’s  horns.  If,  by  some  extra 
massiveness  spontaneously  arising,  or  by  the  formation 
of  an  additional  “ point,”  an  advantage  is  gained  either 
for  attack  or  defence,  then,  if  the  increased  muscularity 
and  strengthened  character  of  the  neck  and  thorax, 
which  wielding  of  these  somewhat  heavier  horns  pro- 
duces, are  in  a greater  or  less  degree  inherited,  and  in 
several  successive  generations  are  by  this  process 
brought  up  to  the  required  extra  strength,  it  becomes 
possible  and  advantageous  for  a further  increase  in  the 
horns  to  take  place,  and  a further  increase  in  the 
apparatus  for  wielding  them,  and  so  on  continuously. 
By  such  processes  only  in  which  each  part  gains 
strength  in  proportion  to  function  can  co-operative 
parts  be  kept  in  adjustment,  and  be  re-adjusted  to  meet 
new  requirements.  Close  contemplation  of  the  facts 
impresses  me  more  strongly  than  ever  with  the  two 
alternatives — either  there  has  been  inheritance  of 
acquired  characters,  or  there  has  been  no  evolution.’* 

As  we  pointed  out  in  the  last  chapter,  there  seems  at 

* Herbert  Spencer,  ' The  Inadequacy  of  Natural  Selection,’ 
p.  20 


72  THE  THEORY  OF  NATURAL  SELECTION 


first  sight  to  be  no  inherent  difficulty  in  the  way  of 
acquired  characters  being  inherited.  Weismann  has, 
however,  pointed  out  a very  serious  difficulty,  which 
is  brought  into  prominence  on  making  a study  of  the 
minute  anatomy  of  the  cells  of  organisms  during  the 
earlier  stages  of  their  development. 

In  the  ordinary  course  of  events  every  one  of  the 
higher  animals  and  plants  begins  its  existence  in  the 
form  of  a single  minute  cell — the  fertilized  ovum  or 
egg.  This  cell  exhibits  no  trace  of  the  complicated 
series  of  organs  which  will  develop  from  it  when  it  is 
subjected  to  the  proper  conditions.  When  the  egg 
is  placed  in  favourable  circumstances  with  regard 
to  warmth,  moisture,  food-supply,  and  the  like,  it 
first  divides  into  two  equal  portions ; and  microscopic 
study  shows  that  elaborate  precautions  are  taken  to 
insure  the  equal  bipartition  of  its  minute  constituent 
parts.  Each  of  the  two  cells  thus  formed  divides 
again  into  two  further  cells,  and  by  a series  of  repeated 
bipartitions  of  this  kind  the  cells  which  constitute  the 
adult  body  are  at  last  brought  into  existence.  Since 
the  body  soon  becomes  differentiated  into  a number  of 
unlike  organs,  it  is  clear  that  at  certain  stages  of  the 
process  the  two  cells  arising  from  a division  must  come 
to  differ  slightly  from  one  another ; and  the  cells  ulti- 
mately produced  show  very  considerable  differences 
of  form,  structure,  and  size.  Among  all  the  cells  which 
finally  arise  those  which  have  undergone  the  least 
modification  from  their  original  condition  are  those 
from  which  are  developed  the  sexual  reproductive  cells, 
or  germ-cells,  of  the  organism.  Indeed,  Weismann  con- 


PURITY  OF  THE  GERM-CELLS 


73 


eludes  that  there  is  no  reason  for  supposing  that  these 
have  undergone  any  modification  at  all. 

If  we  consider  the  cells  which  build  up  an  adult  organ, 
and  for  the  moment  regard  each  separate  cell  as  an 
individual,  we  see  that  each  of  these  individuals 
possesses  an  ancestry  of  cells  stretching  right  back  to 
the  fertilized  ovum — the  single  cell  in  which  the  whole 
organism  originated.  So  far  as  the  later  cell  divisions 
are  concerned,  the  cell-lineage  of  a particular  organ  is 
separate  and  distinct  from  that  of  the  cells  of  any 
other  organ.  At  a certain  distance  back  in  the  history 
of  the  organism  we  shall  come  across  a common  cell- 
ancestor  for  the  cells  belonging  to  a pair  of  neighbour- 
ing organs,  and  the  more  widely  separate  the  parts  to 
which  the  cells  we  are  considering  belong  the  further 
back  must  we  go  before  we  find  their  ancestry  merging 
in  a single  cell.  In  a similar  way  as  with  other  organs, 
so  it  is  found  that  the  sexual  cells  or  germ-cells  of  an 
adult  organism  have  a history  quite  distinct  from  that 
of  the  cells  of  any  other  part  of  the  body ; and  these 
cells  are  the  only  ones  which  are  concerned  in  the 
formation  of  the  offspring.  Thus  we  see  that  the 
particular  cell-lineage  leading  up  to  the  germ-cells  is 
the  only  one  which  is  continued  into  another  genera- 
tion; all  the  others  terminate  with  the  death  of  the 
individual  creature  of  which  they  form  a part.  From 
this  point  of  view  we  may  consider  the  nature  of  a 
given  series  of  animals  as  being  determined  only  by 
the  particular  series  of  cells  which  constitute  the 
direct  ancestry  of  the  germ-cells  in  each  individual ; 
the  cells  which  make  up  the  bodily  structure  are  the 


74  THE  THEORY  OF  NATURAL  SELECTION 

result  of  so  many  offshoots  which  come  to  an  end  at 
the  death  of  the  organism,  and  leave  no  progeny  of 
their  own. 

Wilson  has  expressed  this  view  of  Weismann’s  very 
clearly  ; ‘ It  is  a reversal  of  the  true  point  of  view  to 
regard  inheritance  as  taking  place 
from  the  body  of  the  parent  to  that 
of  the  child.  The  child  inherits 
from  the  parent  germ-cell , not  from 
the  parent  body,  and  the  germ-cell 
owes  its  characteristics  not  to  the 
body  which  bears  it  but  to  its 
descent  from  a pre-existing  germ- 
cell of  the  same  kind-  Thus  the 
body  is  as  it  were  an  offshoot  from 
the  germ-cell.  As  far  as  inheri- 
tance is  concerned  the  body  is 
merely  the  carrier  of  the  germ-cells 
which  are  held  in  trust  for  coming 
generations.’  (The  diagram  illus- 
trating Weismann’s  theory  of  in- 
heritance is  a modification  of  that 
given  by  Wilson.*) 

In  the  light  of  this  conception 
it  may  be  seen  that  the  idea  of  the 
inheritance  of  a modification  ac- 
quired by  an  adult  bodily  organ 
is  comparable  with  the  supposition  that  if  a man 
develops  his  muscles  by  exercise  his  brother’s  children 
will  be  thereby  modified. 

♦ ‘ The  Cell  in  Development  and  Inheritance,’  p.  1 3. 


G,  Germ  cells ; S,  Somatic 
cells. 


PURITY  OF  THE  GERM  CELLS 


75 


The  minute  study  of  the  germ-cells,  taken  in  connec- 
tion with  modern  experimental  work  on  the  methods 
by  which  inheritance  takes  place,  shows  a strong 
tendency  to  confirm  Weismann’s  view,  so  far  as  the  case 
of  distinct  and  definite  characters  is  concerned.  But 
if  we  regard  such  definite  characters  as  having  arisen 
by  definite  steps  or  mutations  according  to  the  view 
now  gaining  ground,  the  study  of  them  will  have  no 
bearing  upon  the  question  of  use-inheritance,  since 
use  does  not  lead  to  large  and  definite  changes  in  the 
individual,  but  to  comparatively  small  changes  of  a 
quantitative  kind. 

There  are  some,  including  de  Vries,  who  regard  all 
fluctuating  variations  (individual  differences)  as  being 
of  the  nature  of  acquired  characters,  and  as  being  at 
the  same  time  capable  of  hereditary  transmission, 
although  de  Vries  believes  the  amount  of  progress 
possible  in  this  way  to  be  strictly  limited.  Let  us  see 
if  there  is  any  way  in  which  a transmission  of  such 
characters  can  be  conceived  of. 

It  must  be  pointed  out  that  the  cells  which  make 
up  an  organism  are  not  completely  marked  off  and 
separated  from  one  another  ; on  the  contrary,  it  seems 
impossible  to  doubt  that  reactions  may  take  place 
between  them  long  after  their  first  formation.  Indeed, 
Sedgwick  has  shown  that  in  a number  of  diverse  kinds 
of  animals  there  is  never  any  sharply  limiting  barrier 
between  cells  at  all,  and  this  writer  has  gone  so  far  as 
to  speak  of  animals  in  general  as  being  built  up  of  a 
continuous  network  of  protoplasm  with  nulcei  at  the 
nodes.  In  plants,  too,  though  at  first  sight  their  con- 


76  THE  THEORY  OF  NATURAL  SELECTION 


stituent  cells  seem  to  be  cut  off  from  one  another  like 
so  many  closed  boxes<  it  has  been  shown  that  there  is 
almost  universal  communication  between  the  proto- 
plasmic masses  so  enclosed,  in  the  shape  of  minute 
fibrils  of  living  substance  which  traverse  the  interven- 
ing walls. 

It  would  thus  seem  possible  for  liquid  or  easily 
soluble  substances  to  pass  freely  from  one  part  of  the 
body  of  an  organism  to  another.  It  is  possible,  for 
example,  supposing  the  enlargement  and  strengthening 
due  to  the  exercise  of  a particular  muscle  to  be  associ- 
ated with  an  increased  production  of  some  definite 
chemical  substance,  to  imagine  that  an  increased 
amount  of  the  same  substance  might  become  enclosed  in 
the  germ-cells,  so  that  this  substance  would  be  present 
in  the  offspring  in  greater  abundance  than  would  have 
been  the  case  if  the  muscle  of  the  parent  had  not  been 
exercised.  And  this  might  facilitate  a further  develop- 
ment of  the  same  muscle  by  exercise  in  the  next 
generation.  In  a similiar  way  increased  bulk  following 
upon  better  nutrition  might  be  inherited,  and  this 
de  Vries  seems  to  have  succeeded  in  showing  to  be 
actually  the  case  in  plants.  Such  changes  might 
normally  be  so  slight  as  to  be  almost  imperceptible  in 
a few  generations,  and  yet  after  marry  generations 
might  accumulate  to  an  important  extent.  It  would 
be  impossible  in  practice  to  distiirguish  changes  of  this 
kind  from  what  are  known  as  accidental  individual 
differences,  and,  indeed,  there  is  no  evidence  at  hand 
to  disprove  de  Vries’  assertion  that  all  continuous 
variations  are  of  the  nature  of  acquired  characters — 


INHERITANCE  OF  ACQUIRED  CHARACTERS  77 


and  we  know  that  continuous  variations  are  in- 
herited. 

On  the  other  hand,  several  lines  of  inquiry  have 
separately  led  to  the  conclusion  that  a great  number 
of  the  visible  characteristics  of  organisms  are  of  a 
definite  kind,  and  are  inherited  definitely,  their  appear- 
ance being  determined  by  the  presence  of  definite 
structures  or  substances  in  the  germ-cells.  The  evi- 
dence, as  we  shall  see  later  on,  points  to  the  conclusion 
that  such  characters  have  arisen  suddenly  at  a single 
step,  and  we  must  conclude  that  in  such  a case  a definite 
change  in  the  germinal  structure  has  been  followed  by 
a definite  alteration  in  the  character  of  the  organism 
arising  from  the  germ  ; since  no  one  can  suppose  that  a 
large  and  definite  structural  alteration  can  be  first 
acquired  by  the  adult  organism  and  then  inherited  by 
its  offspring — such  a process  is  unthinkable. 

Thus  we  see  that  the  inheritance  of  acquired  char- 
acters, if  such  inheritance  really  takes  place  at  all, 
must  be  confined  to  the  transmission  of  changes  of  an 
indefinite  and  quantitative  kind — to  the  case,  in  fact,  of 
continuous  variations  or  individual  differences.  More- 
over, there  is  nothing  to  show  that  all  continuous 
variations  are  not  of  the  nature  of  acquired  characters.* 

♦ We  know,  at  any  rate,  that  continuous  variations  are  'not 
invariably  due  to  the  cause  which  Weismann  supposed — na:feely, 
to  the  mingling  together  of  characters  derived  from  the  twO 
parents — a supposition  which  is  of  fundamental  importance 
to  his  theory — because,  as  Karl  Pearson  has  pointed'  out  in 
this  connection,  parthenogeneticaJly  reproduced  organisms, 
in  which  no  such  mingling  has  taken  place,  may  be  just  as 
variable  as  those  which  owe  their  origin  to  the  process  of 
sexual  generation. 


78  THE  THEORY  OF  NATURAL  SELECTION 


It  is  possible  that  variations  of  this  nature  may  gradu- 
ally lead  to  important  and  even  to  specific  changes, 
but  whether  this  is  the  case  still  remains  to  be  proved. 
On  the  other  hand,  we  shall  see  that  specific  differences 
do  sometimes  arise  at  a single  step,  and  there  is  strong 
but  indirect  evidence  to  show  that  this  is  the  way  in 
which  a very  great  number  of  specific  differences  have 
actually  arisen.  Indeed,  some  have  contended  that 
this  is  the  universal  process  by  which  such  differences 
originate,  but  this  again  is  not  proved,  nor  is  it 
altogether  likely.  In  any  case  the  inheritance  of 
acquired  characters  can  have  nothing  to  do  with  that 
of  definite  and  discontinuous  differences. 

This  is  a problem  to  which  we  shall  return  in  the 
concluding  chapter,  in  the  light  of  further  evidence  con- 
cerning continuous  and  discontinuous  variations  and 
their  manner  of  inheritance,  which  will  be  by  that 
time  available. 


BIBLIOGRAPHY. 

Darwin,  Charles  : The  Origin  of  Species,  sixth  edition,  1872. 
„ „ The  Variation  of  Animals  and  Plants 

under  Domestication,  1868. 

,,  „ The  Descent  of  Man,  1871. 

,,  „ The  Various  Contrivances  by  which 

Orchids  are  Fertilized  by  Insects. 

„ „ Different  Forms  of  Flowers. 

„ „ The  Movements  and  Habits  of  Climbing 

Plants. 

,,  „ Insectivorous  Plants. 

„ „ Life  and  Letters  of  Charles  Darwin. 

„ ,,  More  Letters  of  Charles  Darwin. 

Malthus,  T.  R.  : An  Essay  on  Population. 

Wallace,  A.  R.  j Darwinism. 


BIBLIOGRAPHY 


79 


Weismann,  a.  : Essays  upon  Heredity,  1889. 

,,  The  Evolution  Theory,  1906. 

Spencer,  Herbert  i The  Inadequacy  of  Natural  Selection, 

1893. 

„ „ A Rejoinder  to  Professor  Weismann, 

1893- 

Cope,  E.  D.  : The  Primary  Factors  of  Organic  Evolution,  1896. 
Morgan,  T.  H.  : Evolution  and  Adaptation,  1903. 

„ „ A Critique  of  the  Theory  of  Evolution,  1916. 

Romanes,  G.  J.  : Darwin  and  After  Darwin,  vols.  ii.  and  iii, 
PouLTON,  E.  B.  : Essays  on  Evolution,  1908. 

Punnett,  R.  C.  : Mimicry  in  Butterflies,  1916. 

Darwin,  F.:  Presidential  Address  to  the  British  Association, 
1908. 

Butler,  S.:  Life  and  Habit,  1877. 

,,  Unconscious  Memory,  1880. 

„ Luck  or  Cunning,  1886. 

Willey,  A.:  Convergence  in  Evolution,  iqir. 


CHAPTER  IV 


BIOMETRY 

In  the  present  chapter  we  have  to  consider  in  some 
detail  the  manner  in  which  purely  statistical  methods 
have  been  applied  to  certain  biological  data,  a proceed- 
ing to  which  the  term  biometry  has  been  attached 
by  Professor  Karl  Pearson.  Before  concluding  our 
account  we  shall  give  a brief  sketch  of  some  of  the 
more  important  evidence  bearing  upon  the  problems 
of  evolution  which  has  been  brought  to  light  by  the 
methods  of  biometrical  science. 

The  first  investigator  to  apply  the  methods  of 
statistics  to  the  solution  of  biological  problems  was  the 
Belgian  astronomer,  Quetelet.  In  1845,  in  the  form 
of  a series  of  letters  addressed  to  the  Grand  Duke  of 
Saxe-Coburg  and  Gotha,  Quetelet  published  an  admir- 
able account  of  the  theory  of  probability  and  its 
relation  to  human  affairs,  and  one  in  which  the  use  of 
advanced  mathematics  was  avoided.  The  pioneer  of 
biometry  in  this  country  is  Francis  Galton,  whose  book 
on  ‘ Natural  Inheritance  ’ embodies  an  extremely 
lucid  introduction  to  the  statistical  study  of  variation 
and  inheritance.  From  these  two  works  are  derived 
most  of  the  ideas  submitted  in  the  present  chapter. 
The  more  recent  advances  in  biometry  are  mostly 

80 


BIOLOGY  AND  STATISTICS 


8i 


the  result  of  work  published  by  Professor  Karl  Pearson ; 
they  consist  largely  in  the  elaboration  of  mathematical 
methods  of  dealing  with  statistical  problems,  and  as 
such  it  would  be  inappropriate  to  give  any  further 
account  of  them  here. 

The  mention  of  the  word  • statistics  ’ at  once  raises  a 
certain  prejudice  in  the  ordinary  mind ; in  common 
parlancej  the  unreliability  of  arguments  based  upon 
statistics  is  sometimes  treated  as  proverbial,  and  as 
used  in  biology  they  have,  as  a matter  of  fact,  one  very 
serious  danger  at  least.  Statistics  deal  with  groups 
and  not  with  individuals,  and  there  is  a real  difficulty 
involved  in  the  fact  that  the  average  of  a group  may 
represent  something  quite  different  from  any  individual 
which  the  group  contains,  whilst  at  the  same  time  a 
group  may  include  individuals  of  very  diverse  natures. 
Nevertheless,  when  used  without  prejudice  to  the  future 
examination  of  individual  inheritance  by  more  detailed 
investigations,  the  methods  of  biometry  have  im- 
doubtedly  yielded  information  of  great  value  to  the 
evolutionist,  particularly  in  the  case  of  such  material 
as  that  afforded  by  the  human  race,  since  the  appUca- 
tion  of  precise  experiments  to  this  particular  species  is 
at  present  out  of  the  question. 

Some  students  of  biometry,  however,  would  go  very 
much  further  than  this,  for  it  is  their  professed  opinion 
that  their  own  form  of  study  is  the  only  method  by 
which  any  real  advance  in  our  understanding  of  the 
processes  of  evolution  can  be  brought  about.*  This 
opinion  is  based  upon  the  assumption,  of  which  proof 
*[  This  statement  is  perhaps  no  longer  true. — Ed.] 

6 


82 


BIOMETRY 


is  wanting,  that  new  species  have  arisen  exclusively 
through  the  accumulation  by  natural  selection  of 
variations  of  a strictly  indefinite,  fluctuating,  or  normal 
kind.  We  have  already  seen  reasons  for  believing  that 
this  is  very  far  from  being  the  case,  and  future  chapters 
will  be  found  to  add  considerably  to  the  force  and 
quantity  of  the  evidence  already  adduced. 

Normal  variations,  strictly  speaking,  are  individual 
differences  which  can  be  supposed  to  depend  upon  a 
large  number  of  small  factors  or  causes — factors  so 
numerous  and  so  minute  that  the  numerical  distribu- 
tion of  the  individuals  examined,  when  ranged  in  order 
according  to  the  feature  chosen  for  examination,  is 
found  to  conform  closely  to  that  which  would  be 
expected  on  the  mathematical  theory  of  chance. 
Such  a distribution  will  only  result  when  the  differences 
considered  can  be  strictly  regarded  as  lying  upon  a 
linear  scale,  and  when  they  are  also  evenly  distributed 
along  that  scale.  That  is  to  say,  the  biometrician 
deals  with  continuous  variations  of  a quantitative  kind. 
It  is  to  be  hoped  that  these  somewhat  obscure  sayings 
will  be  more  easily  understood  in  the  light  of  what 
follows. 

The  facts  of  variation  have  not  been  found  readily 
amenable  to  precise  definition,  but  we  shall  endeavour 
to  make  plain  by  the  aid  of  a few  examples  what  kinds 
of  variations  do  and  what  kinds  do  not  appear  to  be 
legitimate  objects  for  the  application  of  biometrical 
methods.  Thus  it  may  be  thought  that  the  biome- 
trician is  outrunning  his  license  when  he  ranks  the 
colours  shown  by  the  iris  of  the  human  eye  in  a con- 


DEFINITE  DIFFERENCES 


83 


tinuous  series  of  eight  shades,  because  in  doing  so  he 
groups  together  a number  of  probably  definite  factors 
with  others  which  are  of  an  indefinite  kind.  When  the 
colours  of  the  human  eye  come  to  be  studied  in  greater 
detail,  there  can  be  little  doubt  that  they  will  be  found 
to  depend  upon  some  such  factors  (among  others)  as 
the  following  ; 

1,  («)  Definite  differences  in  structure,  and  (b)  the 
definite  presence  and  absence  of  pigment  in  certain 
definite  positions  ; as  well  as — 

2.  (a)  Indefinite  variations  (individual  differences)  in 
structure,  and  (b)  in  quantity  of  pigment — if,  indeed, 
the  quantitative  differences  are  not  found  to  be  also 
definite. 

In  the  above  example  a suitable  and  legitimate 
object  for  biometrical  investigation  would  be  the 
differences  in  amount  of  a particular  pigment. 

But  definite  differences  may  also  exist  in  the  case  of 
an  apparently  simple  quantitative  character.  The 
accompanying  figure  (Fig.  2)  shows  the  variations  in 
length  of  the  fruits  of  three  different  but  closely  allied 
species  of  evening  primrose,  as  measured  by  de  Vries. 
In  this  diagram  the  vertical  distances  are  in  each  case 
proportional  to  the  number  of  individuals  having 
particular  lengths  of  fruit,  and  the  actual  length  of  the 
fruit  is  in  each  case  proportional  to  the  horizontal 
distance  from  an  imaginary  vertical  line  some  way  to 
the  left  of  the  figure  ; the  points  thus  plotted  are 
joined  by  straight  lines,  so  that  a polygonal  figure  is 
obtained  representing  the  nature  of  the  variation  in 
each  particular  case.  The  diagram  shows  at  once  that 

6—2 


84 


BIOMETRY 


the  species  A and  C have  each  a characteristic  mean 
size  of  fruit,  and  the  existence  of  this  definite  mean 
is  not  affected  by  the  fact  that  the  range  of  variation 


overlaps  in  all  three  cases.  Species  B,  on  the  other 
hand,  seems  to  show  signs  of  division  into  at  least  two 
separate  groups. 

Differences  of  a similar  kind  are  sometimes  to  be 
found  among  the  progeny  of  the  same  individuals. 
Races  of  garden  peas  may  be  selected  which,  amongst 
other  differences,  are  characterized  by  the  presence  of 
large  and  of  small  seeds  respectively.  In  each  case 
there  is  variation  of  a normal  kind  about  a mean  value, 
but  in  each  case  the  mean  is  quite  distinct.  There  is 
evidence  that  if  a race  of  large-seeded  peas  is  crossed 
with  a small-seeded  variety,  and  the  resulting  cross-bred 
plants  are  self-fertilized,  their  progeny  in  the  second 
generation  will  be  separable  into  different  groups,  and 
some  of  these  will  show  almost  exactly  the  same  size 


DEFINITE  DIFFERENCES 


85 


characteristics  as  those  which  were  exhibited  by  the 
two  original  parental  strains.  The  only  difficulty  in  the 
way  of  invariably  distinguishing  the  two  original  kinds, 
after  their  segregation  in  the  offspring  of  the  cross,  lies 
in  the  fact  that  the  smallest  seeds  of  the  large  type 
may  be  smaller  than  the  largest  seeds  of  the  smaller 
strain,  and  this  is  a difficulty  which  applies  equally 
to  the  original  strains  before  crossing,  as  well  as  to  the 
case  of  the  evening  primrose  fruits  just  mentioned. 

Now,  it  is  clear  that  if  we  mixed  together  the  seeds 
of  several  different  races  of  peas  in  the  proper  pro- 
portions, the  result  might  lead  to  a normal  distribution 
of  the  kind  presently  to  be  described.  The  several 
races,  however,  would  none  the  less  be  perfectly  distinct, 
even  though  we  could  not  separate  the  individual 
seeds  belonging  to  each  by  any  direct  method.*  Such  a 
mixture  of  races  would  constitute  a decided  pitfall  for 
the  unwary  statistician,  and  it  is  well  to  remember 
thatj  after  even  the  most  elaborate  mathematical 
analysis,  the  final  result  cannot  be  clothed  with  any 
greater  amount  of  certainty  than  the  facts  from  which 
the  calculation  set  out.  Those  who  have  made  a large 
expenditure  of  intellectual  effort  in  such  processes  have, 
unfortunately,  a natural  tendency  to  overlook  this 
elementary  fact. 

Prior  to  the  application  of  statistical  methods  to  a 
particular  case  of  normal  variation  a number  of  pre- 
liminary processes  have  to  be  gone  through. 

* It  would  generally  be  possible  to  decide  which  strain  a 
particular  seed  belonged  to  by  sowing  it  and  observing  the 
variation  of  its  offspring. 


86 


BIOMETRY 


Having  selected  a particular  character  for  investiga- 
tion, we  must  make  a quantitative  estimate  of  its 
development  in  each  member  of  a fair  sample  of 
individuals  which  show  the  character  in  question. 
What  is  to  be  understood  as  a fair  sample  was  well 
expressed  by  Quetelet  when  he  wrote  that  statistics 
must  be  collected  without  any  preconceived  ideas, 
and  without  neglecting  any  numbers.  We  shall  find 
that  in  this  point  the  biometrical  method  differs 
from  the  method  introduced  by  Mendel,  since  in  the 
latter  careful  discrimination  of  data  is  an  essential 
feature. 

The  quantitative  determination  of  a character  may 
be  made  either  by  counting  or  by  measurement. 
That  is  to  say,  we  must  proceed  by  measurement  if 
the  character  we  are  dealing  with  is  one  of  size  or  weight, 
and  by  counting  if  the  character  shows  a series  of 
numerical  values  of  its  own — e.g.,  if  it  is  such  a 
character  as  the  number  of  veins  in  a leaf  or  the 
number  of  stigmatic  bands  on  a poppy  capsule. 
Before  we  make  any  determinations  we  ought  to  be 
quite  certain  that  we  are  dealing  with  the  same 
character  in  each  individual,  and  that  the  individuals 
themselves  are  truly  comparable  with  one  another. 
Thus  we  might  make  a series  of  measurements  of  a 
particular  bone  in  a particular  limb  of  a particular 
race  of  human  beings  with  some  assurance  that  we 
should  be  dealing  with  homogeneous  material. 

Our  • measurements  or  countings  will  fall  either 
naturally  or  artificially  into  groups.  In  the  case  of 
countings  the  groups  are  naturally  limited  by  the 


BIOMETRICAL  METHODS 


87 


1 5 10  15  SO  S5  80 


88 


BIOMETRY 


numbers  which  represent  the  character  of  each  indi- 
vidual, whilst  measurements  are  artificially  limited 
through  the  fact  that  they  have  to  be  made  in  units 
of  some  kind— ij.g.,  to  the  nearest  inch  or  some  other 
value.  Such  groups,  characterized  by  equality  of 
range — each,  that  is  to  say,  covering  an  equal  number 
of  units — are  technically  known  as  classes. 

Thus  if  we  are  dealing  with  human  stature,  and  if  our 
measurements  were  made  only  to  the  nearest  inch,  all 
the  individuals  of  6 feet  in  height  would  fall  into  one 
class,  those  of  6 feet  i inch  into  another  class,  and  so 
on.  If,  on  the  other  hand,  we  were  engaged  in  count- 
ing the  number  of  ray  florets  in  the  heads  of  daisies* 
a class  would  include  all  those  heads  which  possessed 
a particular  number  of  rays. 

Without  division  into  classes,  however,  a survey  of 
a comparatively  small  number  of  measurements  may 
be  facilitated  by  ranging  the  values  in  some  kind  of 
order.  This  is  done,  for  example,  in  the  accompanying 
figure  (Fig.  3)  for  the  measurements  to  hundredths  of 
an  inch  of  the  lengths  of  the  body,  wing,  and  tail  of 
thirty-one  specimens  of  a North  American  bird.  The 
diagram  is  taken  from  A.  R.  Wallace’s  ‘ Darwinism.’ 

Even  with  this  small  number  of  measurements  the 
diagram  brings  out  two  points  very  clearly.  In  the 
first  place,  there  is  no  close  correspondence  between  the 
variations  in  length  of  body,  wing,  and  tail.  Secondly, 
in  the  case  of  body-length,  in  respect  of  which  the 
specimens  are  ranged  in  order,  the  number  of  indi- 
viduals of  a medium  size  is  seen  to  be  greater  than  the 
number  of  those  which  show  extreme  values.  This 


NORMAL  VARIABILITY 


89 


excess  of  mediocre  individuals  comes  much  more 
prominently  into  view  as  soon  as  a larger  number  of 
measurements  can  be  considered,  and  the  results 
arranged  in  a different  way. 


Fig.  4. 

The  above  diagram  is  constructed  from  the  entries  in 
the  third  column  of  the  accompanying  table,  which  is 
taken  from  Gabon’s  ‘ Natural  Inheritance.’  It  repre- 
sents the  variations  in  the  strength  of  pull  (as  exerted 
by  an  archer  in  drawing  a bow)  shown  by  519  men 
as  recorded  at  the  International  Health  Exhibition  in 
1884.  Here  equal  distances  measured  off  along  the 
base  line  represent  equal  increments  in  the  strength 
of  pull  of  the  right  hand,  and  the  vertical  heights  of  the 
rectangles  erected  upon  these  bases  represent  the 
percentage  numbers  of  the  men  examined  which 
exhibited  each  value  of  the  character  under  con- 
sideration. In  this  example  it  is  easy  to  see  that  the 
central  class  is  the  largest,  whilst  the  extreme  classes 
contain  a comparatively  small  number  of  individuals. 


90 


BIOMETRY 


TABLE  I.  (FROM  Galton). 

STRENGTH  OF  PULL  (519  MALES,  AGED  23-26). 

From  Records  made  at  the  International  Health  Exhibition 
in  1884. 


Strength  of  Pull. 

Number  of  Cases 
observed. 

Percentages. 

Number  of  Cases 
observed. 

Sums  from 
beginning. 

Under  50  lbs. 

10 

2 

2 

42 

8 

10 

70  » 

J40 

27 

37 

» 80  „ 

168 

33 

70 

))  90  » 

II3 

21 

91 

„ TOO  „ 

22 

4 

95 

Above  TOO  „ 

24 

5 

TOO 

Total 

519 

TOO 

Finally,  we  may  display  in  a somewhat  more  detailed 
fashion  the  result  of  a still  larger  number  of  measure- 


ments. Fig.  5 shows  the  variation  in  stature  of  a large 
number  of  members  of  Cambridge  University  of  British 


NORMAL  VARIABILITY 


91 


extraction,  and  exhibits  in  a concise  form  the  result  of 
4,426  measurements  recorded  by  the  Cambridge  Anthro- 
pometric Society.  In  this  figure  the  stature  in  inches  is 
indicated  on  the  base  line,  whilst  the  perpendicular 
distances  indicate  the  number  of  cases  in  which  each 
particular  height  was  recorded.  The  separate  classes 
in  this  case  include  those  who  were  found  to  fall  within 
the  limits  of  ^ inch  on  either  side  of  each  consecutive 
integral  inch  of  stature,  measurements  which  fell 
exactly  half-way  between  two  classes — e.g.,  one  of 
6gi  inches — being  reckoned  as  a half  to  each  of  the 
classes  in  question.  The  continuous  line  in  the  diagram 
represents  the  form  of  the  ‘ normal  curve  ’ which 
approximates  most  nearly  to  the  line  obtained  by 
joining  together  the  points  actually  plotted. 

There  seems  to  be  good  evidence  that  in  such  a case 
as  that  of  human  stature  the  figure  obtained  in  this  way 
will  approximate  more  and  more  closely  to  the  shape 
of  what  is  known  as  a normal  curve,  according  as  the 
number  of  individuals  measured  and  the  accuracy  of  the 
measurements  increase. 

In  order  to  arrive  at  a proper  understanding  of  this 
fact,  we  must  consider  the  derivation  of  the  ‘ normal  ’ 
curve  from  another  point  of  view — namely,  from  the 
point  of  view  of  the  mathematical  theory  of  proba- 
bility, which  it  will  be  our  endeavour  to  present  in  as 
simple  a manner  as  possible. 

Let  us  consider  the  result  of  tossing  up  a number 
of  similar  coins  simultaneously.  If  we  toss  up  two 
coins  only  we  may  get  any  of  the  following  results  : 
(ij  Head  head,  (2)  head  tail,  (3)  tail  head,  (4)  tail  tail. 


92 


BIOMETRY 


And  It  is  clear  that  any  one  of  these  combinations  is 
equally  likely  to  appear  on  any  given  occasion,  if  the 
coins  are  supposed  to  be  strictly  symmetrical,  and  are 
tossed  up  entirely  at  random.  Now,  the  second  and 
third  results  are  the  same  unless  the  two  coins  are  indi- 
vidually distinguishable.  So  we  may  write  the  most 

ikely  result  of  tossing  up  two  pennies  four  times  in  the 
following  way  : 

1HH+2HT-1-1TT. 

And  in  a similar  way  we  may  discover  that  the  most 
ikely  result  of  tossing  up  three  coins  eight  times  is  : 

I HHH-i-3  HHT-f-3  HTT  + iTTT. 

^ the  first  case  H T is  twice  as  likely  to  appear  as 
H H at  any  single  throw,  and  in  the  second  case  H H T 
IS  three  times  as  likely  as  H H H in  any  single  toss. 

It  IS  possible  to  work  out  the  most  probable  relative 
frequency  of  the  various  possible  combinations  in  the 
case  of  any  number  of  coins.  Thus  for  ten  coins  the 
sequence  of  numbers  runs  : 


TABLE  ll. 


Heads. 

Tails. 

Relative 

— 

Probability. 

10 

0 

I 

9 

8 

7 

1 

2 

3 

10 

45 

120 

0 

5 

4 

3 

4 

210 

5 

6 

252 

210 

7 

120 

2 

j 

8 

45 

9 

10 

0 

I 

NORMAL  VARIABILITY 


93 


These  values  are  plotted  in  the  accompanying 
diagram  (Fig.  6)  as  vertical  distances  above  a base 
line.  The  figure  obtained  by  joining  together  the 
points  thus  arrived  at  may  be  observed  to  show  some 
resemblance  to  the  previous  Figs.  4 and  5. 

The  three  series  of  numbers  already  given  are  those 
which  are  obtained  on  expanding  the  expressions 
(i  + i)*,  (1  + 1)3,  (i  + i)“.  In  general  the  probabilities 
of  the  various  possible  combinations  when  n coins  are 
tossed  simultaneously  are  given  by  the  expanded  value 
of 

Quetelet  has  worked  out  the  relative  probabilities  of 
the  most  frequent  combinations  in  the  case  of  999  coins 
simultaneously  tossed — i.e.,  the  expanded  value  of 
(i  + i)®®®.  A few  of  these  values  are  given  in  the 
following  table : 

TABLE  III. 


Heads. 

Tails. 

Relative 

Probability. 

500 

499 

1*0 

501 

498 

0*996 

502 

497 

0*988 

503 

496 

0*976 

504 

495 

0*961 

505 

494 

0*942 

510 

489 

0*803 

520 

479 

0*432 

530 

469 

0*155 

540 

459 

0*037 

550 

449 

0*006 

560 

439 

0*0006 

It  may  thus  be  seen  that  the  likelihood  that  a result 
appearing  in  any  given  throw  will  show  a stdi  greater 


94 


BIOMETRY 


difference  in  the  relative  number  of  heads  and  tails 
than  560  1 439  becomes  very  small  indeed.  Although 


a throw  of  all  heads  or  all  tails  is  possible,  the  odds 
against  such  a result  being  ever  actually  seen  are 


Fig.  7. 


almost  inconceivably  great.  In  Fig.  7 the  middle 
values  for  (i  + i)®®®  are  plotted — those  combinations 


THE  NORMAL  CURVE 


95 


being  included  which  lie  between  550  : 449  and  449 : 550. 
The  points  thus  obtained  are  so  close  together  that  the 
eye  can  scarcely  distinguish  whether  they  are  joined  by 
straight  or  curved  lines.  We  have,  in  fact,  arrived  at 
a close  approximation  to  the  normal  curve. 

The  curve  thus  approximately  indicated  may  be 
seen  to  be  closely  similar  to  the  one  shown  in  Fig.  5 ; 
in  fact,  the  two  curves  are  of  such  a kind  that  by 
altering  the  vertical  and  horizontal  scales  in  one  of 
the  figures  in  a suitable  ratio  their  form  could  be  made 
practically  identical. 

The  figure  arrived  at  in  this  way  approximates  to  a 
mathematical  curve  which  is  intelligible  to  the  mathe- 
matician from  the  formula  y = e-^.  The  theoretical 
curve  is  really  arrived  at  by  supposing  n in  the  ex- 
pression (i  +1)"  to  become  indefinitely  great.  Prac- 
tically, by  making  n very  large  we  can  get  as  near  an 
approximation  as  we  may  wish  to  the  normal  curve  of 
theory.  Even  in  the  case  of  relatively  small  values  of 
n the  approximation  to  the  normal  curve  is  fairly  close, 
as  may  be  seen  by  comparing  together  Figs.  6 and  7. 

The  example  of  tossing  up  coins  was  only  taken 
as  a means  of  illustrating  the  more  general  assumption 
of  an  event  or  a magnitude  depending  upon  a number 
of  causes  of  equal  strength,  which  in  the  long-run  act 
with  equal  frequency  in  two  opposite  directions.  We 
can  understand  that  human  stature  may  afford  a 
comparable  case,  when  we  consider  the  large  number 
of  bones  and  cartilages  the  lengths  of  which  must  be 
added  together  in  order  to  make  up  the  total  stature 
of  any  individual,  and  that  the  separate  length  of  each 


96 


BIOMETRY 


one  of  these  elements  depends  upon  factors  which  we 
have  no  means  of  classifying  exactly. 

It  now  becomes  necessary  to  mention  one  or  two 
technical  terms  which  are  used  in  connection  with  the 
normal  curve.  The  mode  of  such  a curve  is  the 
longest  perpendicular  which  can  be  drawn  from  the 


base-line  to  meet  the  curve  itself  (M  in  the  above 
figure).  The  curve  is  symmetrical  on  either  side  of  the 
mode — that  is  to  say,  any  two  perpendiculars  drawn 
from  the  base  to  the  curve  on  either  side  of  the  mode 
and  at  the  same  distance  from  it  will  be  equal  in 
length. 

When  dealing  with  a symmetrical  curve  the  position 
of  the  mode  is  identical  with  that  of  the  median — the 
perpendicular  line  which  divides  the  area  of  the 
curve  into  two  equal  halves,  and  the  foot  of  this 
perpendicular  also  represents  the  mean  or  average  of 


THE  NORMAL  CURVE 


97 


all  the  values  from  which  the  curve  is  constructed.  In 
any  actual  case  obtained  by  practical  methods  the 
position  of  the  mode,  the  median,  and  the  mean  will 
only  be  approximately  the  same,  because  such  a curve 
is  never  perfectly  symmetrical. 

The  same  curve  can  always  be  reconstructed  if  the 
position  and  magnitude  of  the  mode  are  known,  and, 
in  addition,  any  one  other  point  on  the  curve  itself. 
A convenient  point  to  take  for  this  purpose  is  the  point 
at  which  the  curve  is  met  by  a straight  line  erected 
perpendicular  to  the  base  at  such  a distance  from  the 
median  that  it  divides  the  area  enclosed  by  the  median, 
the  base,  and  half  the  curve  into  two  equal  parts. 
The  distance  of  such  a perpendicular  from  the  median 
is  known  as  the  quartile.  Any  given  curve  will  have 
two  quartiles  one  on  either  side  of  the  median ; they 
are  shown  at  Q and  Q'  in  Fig.  8. 

In  practice  an  approximation  to  the  normal  curve  of 
variability  is  constructed  by  plotting  the  values  of  a 
number  of  separate  measurements  or  other  determina- 
tions made  upon  different  individuals.  A variate  is 
one  of  the  separate  numerical  values  from  which  a 
curve  of  variability  can  be  constructed  ; the  biome- 
trician usually  deals  with  some  such  number  as 
1,000  variates.  The  total  number  of  variates  is 
represented  by  the  area  enclosed  by  the  curve,  and 
it  will  be  seen  that  half  the  total  number  of  variates 
falls  between  the  two  quartiles  and  half  outside 
them. 

A class  (cf.  p.  88)  may  be  defined  as  j.  group  of 
variates  all  of  which  show  a particular  value  or  a value 

7 


98 


BIOMETRY 


falling  between  certain  limits.  The  frequency  of  a class 
is  the  number  of  variates  which  it  contains. 

The  amount  of  variation  shown  by  a particular 
group  of  variates  is  measured  by  the  degree  of  slope 
of  the  curve.  A flat  curve  indicates  greater  variability 
and  a steep  curve  denotes  less  variability.  The  flatter 
the  curve — supposing  the  area  (the  number  of 
variates)  to  remain  the  same — ^the  further  from  the 
mode  will  be  the  position  of  the  quartile,  so  that  the 
distance  of  the  quartile  from  the  mode  may  be  taken 
as  a convenient  measure  of  variability.  In  a theoreti- 
cally perfect  curve  the  distance  of  Q and  Q'  from  M 
is  equal.  A curve  obtained  from  an  actual  series  of 
variates  is  never  perfectly  symmetrical,  so  that  in 
practice  the  distance  of  Q and  Q'  from  M may  not  be 
quite  the  same.  In  such  a case  the  average  of  the 
two  distances  is  taken  as  the  measure  of  the  variability 
of  the  material  in  question,  and  this  value  may  be 
briefly  denoted  by  the  letter  q. 

In  the  example  of  variability  of  stature  represented 
by  Fig.  5,  q is  equal  to  i’6  inches.  This  amount  of 
variability  can  therefore  be  compared  with  other 
values  representing  the  variability  in  statiire  and  in 
other  characters  shown  by  various  other  groups  of 
individuals.  This,  then,  is  the  first  important  biome- 
trical result  which  we  have  arrived  at — the  determina- 
tion of  a numerical  value  representing  the  amount  of 
normal  variability  in  any  given  case. 

A measure  of  variability  more  often  used  than  the 
quartile,  especially  in  recent  work,  is  what  is  known 
as  the  standard  deviation  of  a normal  curve,  and  may 


PROBABLE  ERROR 


99 


be  expressed  shortly  as  <j.  <r  represents  a distance  from 
the  mode  equal  to  0-6745.  Thus  if  a is  known,  q 
can  be  readily  determined,  and  vice  versa.  The  reason 
for  the  more  frequent  use  of  a-  is  that  it  happens  to  be 
determinable  with  greater  accuracy  from  an  actual 
series  of  variates.* 

We  have  still  to  find  a measure  which  will  enable  us 
to  compare  the  variability  of  parts  or  organisms  so 
different  that  they  require  to  be  expressed  in  units  of 
quite  different  magnitudes.  For  this  purpose  what  is 
known  as  the  coefficient  of  variability  is  used.  This  is 
a purely  abstract  number  obtained  by  dividing  the 
standard  deviation  by  the  magnitude  of  the  mean  in 
any  particular  case,  and  multiplying  the  result  by  100. 
In  this  way  a measure  of  variability  is  arrived  at  inde- 
pendent of  the  particular  kind  of  units  of  measurement 
which  were  employed  in  obtaining  it,  and  variabilities 
previously  expressed  in  terms  of  different  units  can 
thus  be  compared  together. 

The  circumstance  that  half  the  total  number  of 
variates  lies  outside  the  limits  of  the  quartiles  and  half 
within  leads  us  to  the  consideration  of  what  is  known 
as  the  probable  error.  The  probable  error  of  any 
statistical  determination  is  obtained  by  finding  a pair 

* o-  is  found  by  multiplying  the  square  of  the  deviation  of  each 
class  from  the  mean  (or  mode)  by  the  frequency  of  the  class, 
adding  together  the  series  of  products  so  obtained,  dividing 
this  number  by  the  total  number  of  variates,  extracting  the 
square  root  of  the  result,  and  multiplying  by  the  number  of 
units  in  the  class  range  (this  last  number  is  very  often  unity). 
For  further  details  with  regard  to  the  properties  of  the  normal 
curve  Daven4?ort’s  ‘ Statistical  Methods  ’ may  be  consulted 

7—2 


100 


BIOMETRY 


of  values  lying  one  above  and  one  below  the  true  value 
required — e.g.,  the  average  stature  of  the  whole  of  a 
race — such  that  it  is  an  even  chance  that  the  value 
actually  found  will  lie  between  them.  Or  the  same 
thing  may  be  expressed  in  another  way.  . If  we  plot  in 
the  form  of  a curve  a long  series  of  actual  determina- 
tions of  a particular  value,  the  probable  error  of  a 
single  determination  will  be  nearly  equal  to  the 
quartile  of  the  curve  so  obtained.  We  may  illustrate 
this  state  of  things  from  our  example  of  tossing  coins, 
or  still  better  by  the  essentially  similar  case  of  draw- 
ing balls  out  of  a bag  which  contains  a very  large 
number  of  balls — black  and  white  in  equal  numbers. 
Here  the  value  to  be  determined  experimentally  is 
the  relative  number  of  black  balls  to  white,  which  we 
know  as  a matter  of  fact  to  be  equality;  and  our 
single  determination  may  consist  in  drawing  out  a 
hundred  balls,  which  are  afterwards  returned  to  the 
bag.  If  we  do  this  i,ooo  times,  and  plot  the  number 
of  black  balls  drawn  each  time,  we  shall  arrive  approxi- 
mately at  a curve  having  its  mode  at  50,  and  possessing 
a standard  deviation  which  it  is  possible  to  determine 
from  the  instructions  given  in  the  footnote  to  p.  99. 
Multiplying  a by  0-6745  gives  us  the  quartile,  which 
represents  the  probable  error  of  a single  determination. 
That  is  to  say,  it  is  an  even  chance  whether  any  single 
determination  differs  from  50  by  more  or  less  than  q. 
In  this  particular  example  the  quartiles  would  be  found 
to  lie  very  nearly  at  46-6  and  53'4>  so  that  the  value  of 
the  probable  error  is  3-4. 

The  properties  of  the  normal  curve  tell  us  a number 


PROBABLE  ERROR 


lOI 


of  useful  things  about  the  probable  error.  In  the  first 
place  its  value  varies  inversely  as  the  square  root  of 
the  number  of  variates — that  is  to  say,  that  in  such  a 
case  as  we  have  just  described  the  probable  error  varies 
inversely  as  the  square  root  of  the  number  of  balls 
drawn  each  time.  We  can  realize  this  point  more 
clearly  when  we  remember  that  the  linear  dimensions 
of  a curve  vary  with  the  square  root  of  its  area  (the 
number  of  variates) ; the  accuracy  of  our  determination 
varies  in  fact  with  the  quartile,  which  is  the  linear 
distance  from  the  mode  of  a certain  perpendicular. 

We  have  seen  that  it  is  an  even  chance  whether  a 
single  determination  differs  from  the  proper  value  by 
more  or  less  than  the  amount  of  the  probable  error, 
an  amount  which  we  may  denote  by  the  letter  e. 
The  chance  that  any  particular  determination  differs 
from  the  true  value  by  more  than  twice  the  probable 
error  is  4-5  to  i against. 

The  chance  that  it  differs  by  more  than  3^  is  21:1  against, 
w w » 4*^  142  : I ,, 

» ))  M » 5^  >>  ^>3^0  : I ,, 

This  is  clearly  very  valuable  information  to  possess 
when  we  are  dealing  with  any  kind  of  statistics. 

We  must  now  pass  on  to  consider  what  methods  are 
available  to  the  biometrician  for  deahng  with  the 
problems  of  heredity.  His  way  is  to  take  a large 
number  of  pairs  of  relations,  each  pair  consisting,  say, 
of  a father  and  a son,  and  to  find  out  how  much  more 
like  the  members  of  such  a pair  are  to  one  another  on 
the  average  than  the  members  of  similar  pairs  of 
individuals  would  be,  if  taken  at  random  and  without 


TABLE  IV. 


103 


BIOMETRY 


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Total  sons  for  fathers! 

of  each  height  ...J 
Modes  of  arrays  ofl 
sons  J 

CORRELATION 


103 


regard  to  relationships  from  among  the  general 
population  to  which  these  fathers  and  sons  belonged. 

Now  we  shall  see  later  on  that  this  is  not  the  only 
way  of  looking  at  the  phenomenon  of  heredity,  nor  is 
it  the  way  which  is  most  familiar  to  biologists.  But 


62  64-  66  68  70  72  74-  76 


Fig.  9.— Diagram  of  Correlation. 


it  is  important  to  remember  that  what  the  biometrician 
means  by  amount  of  inheritance  is  a numerical  value 
which  expresses  the  average  degree  of  likeness  between 
a particular  pair  of  relatives — for  example,  fathers  and 


sons. 


104 


BIOMETRY 


In  the  * correlation  table  ’ given  on  p.  102  — a 
purely  imaginary  illustration — there  are  tabulated  the 
statures  of  4,503  fathers,  and  those  of  one  son  of 
each  of  them.  Thus  14  fathers,  each  62  inches  high, 
are  supposed  to  have  had  14  sons,  whose  heights  are 
given  in  the  first  column.  The  series  of  heights  of  sons 
corresponding  to  a particular  class  of  fathers  is  known 
as  an  array.  Thus  each  column  of  the  table  represents 
an  array  of  sons,  and  similarly  each  line  represents  an 
array  of  fathers.  The  mode  of  each  array  of  sons  is 
given  in  the  bottom  line  of  the  table. 

Now  if  sons  were  on  the  average  exactly  the  same 
height  as  their  fathers,  the  modal  value  of  each  array 
of  sons  would  be  the  same  as  the  height  of  the  corre- 
sponding class  of  fathers.  If,  on  the  other  hand,  there 
were  no  correlation  between  the  heights  of  sons  and 
those  of  their  fathers  the  mode  of  every  array  of  sons 
would  be  the  same,  and  this  value  would  be  identical 
with  the  mode  of  the  heights  of  all  the  sons  taken 
at  once.  The  actual  result  is  found  to  be  intermediate 
between  these  two  possible  extremes.  Thus  we  see 
that  sons  tend  to  be  like  their  fathers  in  respect  of 
stature,  but  not  exactly  like,  cind  if  the  example  given 
were  a real  one  the  fundamental  fact  of  a positive 
resemblance  or  correlation  between  the  statures  of 
fathers  and  sons  would  at  once  be  clearly  established. 

The  way  in  which  a numerical  value  is  attached  to 
this  correlation  can  be  shown  graphically. 

In  the  diagram  shown  in  Fig.  9,  tfih  dots  indicate 
the  values  of  the  modes  of  the  several  arrays  of  sons 
as  read  off  on  the  vertical  scale  to  the  left  of  the  figure. 


CORRELATION 


105 


the  heights  of  the  corresponding  classes  of  fathers 
being  read  off  on  the  horizontal  scale.  It  will  be  seen 
that  this  series  of  dots  lies  nearly  in  a straight  line 
which  is  inclined  at  a certain  angle  to  the  horizontal. 

Now  if  there  were  perfect  correlation  between  the 
heights  of  fathers  and  sons,  and  no  tendency  existed 
for  sons  to  be  more  like  the  general  mode  of  the  popula- 
tion than  their  fathers  are,  the  inclination  of  the  line 
obtained  in  the  above  manner  would  be  one  of  45 
degrees,  as  in  the  case  of  the  line  CD  which  passes 
through  the  points  at  which  the  values  as  read  off 
in  the  vertical  and  horizontal  scales  are  identical.  If, 
on  the  other  hand,  there  were  no  correlation  the  line 
would  be  horizontal,  as  EF. 

The  value  taken  to  represent  the  amount  of  correla- 
tion is  the  degree  of  slope  of  the  line  AB.  This  is 
expressed  mathematically  as  tan  a,  a being  the  angle 
which  the  line  in  question  makes  with  the  horizontal. 

When  there  is  positive  correlation  this  angle  falls 
between  o and  45  degrees,  and  tan  a between  o and  i. 
In  the  present  instance  tan  a is  0-5.  This  value  is 
known  as  the  coefficient  of  correlation,  and  affords 
the  basis  of  a numerical  comparison  with  other  similar 
coefficients  obtained  for  other  characters  besides 
stature,  and  in  the  case  of  other  pairs  of  relatives 
besides  fathers  and  sons. 

It  ought  now  to  be  clearly  understood  that  a com- 
plete resemblance  between  each  class  of  fathers  of  a 
particular  stature  and  the  average  stature  of  the 
corresponding  array  of  sons  would  be  indicated  by 
the  close  approximation  of  our  plotted  points  to  a 


io6 


BIOMETRY 


straight  line  making  an  angle  of  45  degrees  with  the 
base  line — a line,  that  is  to  say,  having  a slope  of 
I in  I,  or  unity ; whilst  the  entire  absence  of  correlation 
would  be  represented  by  a line  having  no  slope — that 
is  to  say,  a horizontal  line.  The  actual  result  in  the 
example  given  is  represented  by  a line  having  a slope 
of  nearly  i in  2,  or  0-5.* 

In  the  following  table  there  are  set  down  the  corre- 
lation coefficients  for  stature  in  the  case  of  seven  pairs 
of  relations,  as  obtained  from  actual  data  of  a similar 
character  to  that  already  given  by  way  of  illustration. 


TABLE  V.  (from  Pearson). 

CORRELATION  COEFFICIENTS  FOR  HUMAN  STATURE. 


Father  and  son 

...  0-514 

Father  and  daughter  ••• 

...  0-510 

Mother  and  son 

...  0-494 

Mother  and  daughter... 

...  0-507 

Brother  and  brother  ... 

...  0-5 II 

Sister  and  sister 

...  0-537 

Brother  and  sister 

- 0-553 

Of  the  above,  the  first  four  values,  representing 
correlation  between  parents  and  children,  are  seriously 

* Correlated  Variability, — A precisely  similar  method  is 
employed  to  measure  the  corrdation  of  two  parts  or  organs  of 
the  same  individual.  For  example,  the  lengths  of  the  right 
and  left  arms  of  men  are  very  closely  correlated.  In  order  to 
attach  a numerical  value  to  this  correlation  the  lengths  of 
the  right  arms  of  a number  of  men  are  treated  in  the  same 
way  as  the  statures  of  fathers  in  the  example  given,  and  the 
lengths  of  their  left  arms  in  the  same  way  as  the  statures  of 
sons.  The  proper  correlation  coefficient  can  then  be  found 
by  plotting  the  result ; or  the  labour  of  plotting  may  be 
obviated  by  a process  of  calculation. 


CORRELATION 


107 


affected,  as  regards  their  value  for  representing  quanti- 
tatively a hereditary  relationship  between  two  indi- 
viduals, by  the  fact  that  the  statistics  from  which  they 
are  derived  show  the  existence  of  a marked  correlation 
between  husbands  and  wives  in  point  of  stature, 
amounting,  indeed,  to  as  much  as  0-28 — the  result 
of  what  is  technically  described  as  selective  mating. 
In  the  absence  of  such  a relation  between  the  statures  of 
the  parents,  the  correlation  between  parent  and  child 
might  be  expected  to  be  distinctly  less  than  that 
between  pairs  of  brothers  or  sisters. 

The  term  correlation  replaces  to  some  extent  the 
older  term  regression  employed  by  Galton.  WTien 
speaking  of  regression  the  facts  already  described  are 
regarded  from  a slightly  different  point  of  view.  It 
is  sometimes  found  convenient  to  speak  of  the  regres- 
sion of  the  mean  stature  of  an  array  of  sons  toward 
the  mean  of  the  general  population,  instead  of  speaking 
of  the  correlation  between  the  filial  mean  and  the 
value  of  the  parental  class. 

Regression  represents  the  extent  to  which  the 
average  son  is  more  like  the  mean  of  the  general 
population  than  his  father  is.  Correlation,  on  the 
other  hand,  indicates  the  amount  by  which  the  son  is 
more  like  his  parent  than  he  is  to  the  average  of  the 
general  population.  Thus,  instead  of  being  exactly 
like  their  parents,  children  are  said  to  show  regression 
towards  the  mean  of  the  general  population  to  which 
both  parents  and  children  belong. 

In  a case  where  the  mean  height  of  the  fathers  is 
identical  with  the  mean  height  of  the  sons  examined. 


io8 


BIOMETRY 


and  both  are  the  same  as  the  mean  height  of  the  general 
population,  the  coefficient  of  regression  is  simply  equal 
to  the  reciprocal  of  the  correlation  coefficient  between 
fathers  and  sons.  In  actual  practice  this  condition  is 
seldom  realized,  and  it  is  then  necessary  to  use  a more 
elaborate  method  in  order  to  determine  the  value  of 
the  regression  coefficient. 

j Professor  Pearson  has  extended  the  idea  of  correla- 
tion to  the  case  of  characters  which  are  not  capable  of 
exact  quantitative  measurement.  This  extension  is 
based  upon  the  assumption  that  such  characters  foUow 
a normal  law  of  distribution  in  their  variation,  just  in 
the  same  way  as  such  a character  as  human  stature  was 
found  to  do.  There  is  considerable  doubt  as  to  how 
far  this  assumption  is  justified,  so  that  at  the  outset 
we  may  feel  disposed  to  attach  less  importance  to  the 
actual  values  arrived  at  in  this  way  than  we  should  in 
the  case  of  characters  which  can  be  shown  to  vary 
normally.  The  method  of  calculation  actually  em- 
ployed involves  somewhat  comphcated  mathematical 
processes,  but  on  Professor  Pearson’s  authority  we 
may  assume  both  the  validity  of  the  method  and  the 
accuracy  of  the  results  obtained — ^so  far  as  the  actual 
process  of  computation  is  concerned.  For  the  purpose 
of  making  the  necessary  calculations  the  data  were 
arranged  in  such  a form  as  in  the  table  on  p.  109. 

By  the  suitable  treatment  of  these  figures  the 
value  0-45  was  obtained  as  representing  the  coefficient 
of  correlation  between  sire  and  filly. 

The  amount  of  reliance  which  is  to  be  placed  in  the 


CORRELATION 


109 

above  method  of  determining  the  value  of  a correlation 
coefficient  was  tested  by  arranging  in  a similar  manner 
data  with  regard  to  stature  which  had  already  been 
treated  in  the  form  of  a complete  correlation  table. 
The  whole  number  of  fathers  was  divided  into  two 
groups  containing  the  individuals  above  and  below  a 
certain  stature,  and  the  same  was  done  in  the  case 
of  the  sons.  And  the  separation  into  two  groups  was 
made  in  several  different  ways  by  taking  the  dividing 


PARENTAL  CORRELATION  OF  COAT  COLOURS  IN  HORSES. 


Fillies. 

Sires. 

1 

> 

Total. 

1 

Colour. 

Bay 

and  Darker. 

Chestnut  and 
Lighter 

Bay  and  darker 

631 

125 

. 1 

756 

Chestnut  and  lighter 

147 

147 

294 

Total  

778 

272 

line  between  the  groups  at  various  heights.  By 
applying  to  the  statistics  disposed  in  these  various 
arrangements  the  same  method  as  w'as  applied  to  the 
statistics  of  horse  colour  already  referred  to,  values 
varying  between  0-52  and  o-6  were  obtained  for 
parental  correlation ; whereas  the  value  arrived  at  by 
the  more  usual  and  reliable  method  was  0‘5i4.  It 
would  therefore  appear  that  there  is  with  this  method 
a tendency  to  obtain  too  high  a figure,  as  compared  with 
that  derived  from  the  method  of  the  complete  correla- 
tion table.  When  this  source  of  inaccuracy  is  taken 


110 


BIOMETRY 


into  consideration,  in  combination  with  the  doubtful- 
ness of  the  assumption  upon  which  the  method  is 
based,  it  seems  clear  that  its  use  will  only  give  us  a 
roughly  approximate  view  of  the  correlation  actually 
existing  in  the  cases  to  which  it  is  applied.  Having 
made  this  reservation,  we  may  compare  the  values 
given  in  the  following  table  with  those  which  appeared 
in  Table  V. : 


TABLE  VI.  (from  Pearson). 

AVERAGE  PARENTAL  CORRELATION. 

Human  eye  colour  ...  ...  ...  0*495 

Horse,  coat  colour  ...  ...  ...  0*522 

Basset  hound,  coat  colour  ...  ...  0*524 

Greyhound,  coat  colour  ...  ...  0*507 


AVERAGE  FRATERNAL  CORRELATION. 


Human  eye  colour  ...  ...  ...  0*475 

Horse,  coat  colour  ...  «..  ...  0*633 

Basset  hound,  coat  colour  ...  ...  0*524 

Greyhound,  red  in  coat  ...  ...  0*700 

Greyhound,  black  in  coat  ...  ...  0*660 


Thus  if  we  use  the  term  inheritance  at  present  simply 
to  express  the  fact  that  a more  or  less  definite 
numerical  value  can  be  attached  to  the  average  amount 
of  resemblance  between  any  specified  pair  of  relatives, 
we  see  that  a considerable  number  of  physical  characters 
appear  to  be  inherited  at  approximately  the  same  rate 
in  men  and  in  animals. 

More  than  this,  Professor  Pearson  has  shown,  by 
the  use  of  the  same  method  as  was  applied  to  the  case 
of  physical  characters  not  quantitatively  measurable, 
that  the  average  resemblance  in  mental  characteristics 


MENTAL  CHARACTERS 


III 


between  pairs  of  brothers,  pairs  of  sisters,  and  pairs 
made  up  of  a brother  and  a sister,  can  be  expressed 
by  the  values  given  in  the  following  table  : 


TABLE  VII.  (from  Pearson). 


Character. 

Brothers. 

Sisters. 

Brother  and 
Sister. 

Vivacity  

0*47 

0*43 

0*49 

Assertiveness 

0*53 

0*44 

0*52 

Introspection 

0*59 

0*47 

0*63 

Popularity  

o‘5o 

0*47 

0*49 

Conscientiousness  ... 

0*59 

0*64 

0-63 

Temper  

0*51 

0*49 

0-51 

Ability  

0*46 

0*47 

0*44 

Handwriting 

0*53 

0*56 

048 

Mean 

0*52 

0*51 

0*52 

A sample  of  the  collected  facts  from  which  this 
information  is  arrived  at  is  given  in  the  following 
table : 


CONSCIENTIOUSNESS  : BROTHER— BROTHER. 


First  Brother. 

Second  Brother. 

Total. 

Keen. 

Dull. 

Keen 

970 

2i6’5 

i,i86'5 

Dull 

2i6*5 

287 

5o3'5 

Total 

i,i86’5 

5o3’5 

1,690 

Every  child  was  classified  in  this  way  as  being  either 
above  or  below  an  average  standard  in  respect  of  each 


II2 


BIOMETRY 


character.  The  estimations  were  made  by  teachers 
having  at  least  six  months’  experience  of  the  children 
in  question. 

The  method  of  statistical  treatment  was,  as  we  have 
said,  the  same  as  that  employed  in  the  case  of  physical 
characters  not  capable  of  quantitative  measurement, 
and  there  is  little  doubt  that  it  is  equally  valid  in  the 
present  case.'  We  may  well  feel,  however,  some 
hesitation  in  accepting  as  sound  the  data  to  which  the 
method  is  applied.  At  the  best  this  data  can  only  be 
of  a roughly  approximate  kind.  The  evidence  is, 
however,  undoubtedly  sufficient  to  establish  the  con- 
clusion that  mental  characters  are  inherited  in  man, 
and  that  they  are  probably  inherited  at  a rate  not 
greatly  different  from  that  at  which  physical  characters 
are  inherited.  For  it  will  be  observed  that  the  values 
given  in  Table  VII.  are  in  close  agreement  with  one 
another,  and  that  they  also  agree  with  the  average 
value  of  fraternal  correlation  as  found  for  a variety  of 
physical  characters  both  in  men  and  in  other  animals. 
Assuming — and  the  assumption  seems  to  be  a reasonable 
one — that  equal  fraternal  correlations  indicate  the  exist- 
ence of  equal  correlations  between  parents  and  children, 
we  arrive  at  the  conclusion  that  the  resemblance 
between  parents  and  their  offspring  is  of  much  the 
same  kind  and  amount  in  the  case  of  mental  as  it  is  in 
the  case  of  bodily  characteristics. 

What  we  may  perhaps  describe  as  the  main  general- 
ization so  far  arrived  at  by  biometricians  is  known 
as  the  Law  of  Ancestral  Heredity.  This  hypothesis 
supposes,  or  at  least  in  its  original  form  supposed,  that^ 


LAW  OF  ANCESTRAL  HEREDITY 


113 


every  ancestor  of  a particular  individual  contributes 
its  quota  to  the  heritable  qualities  displayed  by  that 
individual.  The  law  also  states  that  the  average 
amount  of  resemblance  between  an  individual  and  any 
particular  ancestor  is  capable  of  definite  numerical 
expression.  Thus  the  mean  amount  of  correlation 
between  (i)  the  two  parents  and  the  offspring,  (2)  the 
four  grandparents  and  the  offspring,  (3)  the  eight 
great-grandparents  and  the  offspring,  and  so  on,  is 
believed  to  diminish  in  a geometrical  series,  which  is 
the  same  for  all  organisms  and  for  all  characters.  The 
actual  amounts  of  these  correlations  were  expressed  by 
Galton  in  the  form  of  the  series  0-50,  0-25,  0‘i25,  etc. 
Pearson  regards  them  as  being  more  nearly  represented 
by  the  more  rapidly  diminishing  series  0-6244,  O'IQSS, 
0-0630,  etc. 

Now,  there  can  be  no  doubt  that  the  law  as  stated 
above  has  been  disproved  in  specific  instances,  and  was 
indeed  disproved  by  the  work  of  Gregor  Mendel  before 
ever  it  was  enunciated,  although  Mendel’s  work  was 
not  generally  known  until  later.  According  to  Mendel’s 
theory  of  inheritance,  certain  ancestors  contribute 
nothing  to  the  constitution  of  certain  offspring  in  respect 
of  certain  characters.  Furthermore,  the  modification  of 
the  law  of  ancestral  heredity  which  apphed  to  alterna- 
tive inheritance,  and  which  was  assumed  in  working 
out  the  inheritance  of  coat  colour  in  thoroughbred 
horses,  has  since  been  shown  not  to  apply  to  that 
particular  case. 

Unfortunately,  most  of  the  further  biometrical 
generalizations  which  concern  students  of  heredity 

8 


BIOMETRY 


114 

are  based  upon  the  assumption  that  the  law  of  ancestral 
heredity  is  strictly  true.  So  that  whilst  we  have 
spent  some  time  in  considering  the  facts  of  normal 
variability  and  of  correlation  between  relatives, 
because  these  facts  are  quite  independent  of  any 
theoretical  assumption,  the  remainder  of  our  review 
must  be  passed  over  at  a more  rapid  rate.  Until 
the  theoretical  conclusions  now  to  be  described  have 
been  revised  by  their  authors  in  the  light  of  recent 
knowledge,  it  is  difficult  to  say  how  much  reliance  is  to 
be  laid  upon  them,  but  it  seems  quite  likely  that  they 
will  hold  good  as  approximations.  Indeed,  though 
not  applying  to  individual  cases,  the  law  of  ancestral 
heredity  does  seem  to  hold  good  as  a statistical  state- 
ment of  general  results,  so  that  there  would  be  no  objec- 
tion to  it  on  either  theoretical  or  practical  grounds  if 
only  it  had  been  enunciated  in  some  such  terms  as  ‘ a 
law  of  average  ancestral  resemblance.’  Thus  it  is  quite 
possible  that  the  total  contribution  of  the  eight  great- 
grandparents  of  an  individual  may  be  on  the  average 
correctly  represented  by  Pearson’s  fraction,  even 
though  their  individual  contributions  are  not  always 
the  same. 

Let  us,  then,  briefly  examine  some  of  the  further 
conclusions  which  have  been  drawn  from  the  data  of 
the  biometricians. 

Assuming  the  law  of  ancestral  heredity,  Pearson  has 
arrived  at  very  interesting  conclusions  with  regard  to 
the  effects  of  artificial  selection  when  the  correlation 
coefficients  have  those  values  which  have  been  actually 
found  for  them  in  the  case  of  the  human  race.  In  the 


EFFECT  OF  SELECTION 


115 

statement  which  follows,  ancestors  are  supposed  to 
have  been  selected  showing  in  each  generation  a devia- 
tion h from  the  general  mean  of  the  population.  Thus, 
suppose  the  character  selected  to  be  stature  j suppose 
the  mean  height  of  the  population  to  be  6 feet,  and 
the  selected  individuals  to  be  6 feet  6 inches  high ; h is 
then  6 inches,  and  only  individuals  of  a height  of 
6 feet  6 inches  would  be  selected  as  parents  in  each 
generation,  so  that  after  three  generations  of  selection 
we  should  be  dealing  with  children  whose  parents, 
grandparents,  and  great-grandparents  were  all  of  this 
particular  height. 

Pearson  calculates  that  after  one  generation  of 
selection  the  immediate  offspring  will  show  0'62  of 
the  character  selected  (0-62  h).  After  two  generations 
they  will  show  0-82  h,  after  three  0-89  h,  and  after  a 
great  number  of  generations  0 92  h.  Thus  in  a com- 
paratively small  number  of  generations  the  development 
of  a character  may  be  raised  to  within  90  per  cent, 
of  the  value  selected,  but,  after  this,  further  selection  has 
very  little  effect.  If  selection  is  stopped  after  one 
generation,  and  the  selected  stock  is  then  inbred,  it 
was  calculated  that  the  first  generation  of  inbred  stock 
would  show  0-59  h,  the  second  0 56  h,  the  third  0^52  h, 
and  the  tenth  0-35  h.  If,  on  the  other  hand,  in- 
breeding  w'as  started  after  the  selection  had  continued 
for  a large  number  of  generations,  the  first  generation 
of  inbred  stock  will  show  o-86  h,  the  second  o-8i  h, 
the  third  077  h,  and  the  tenth  0-51  A.  So  that  in- 
breeding  of  a selected  stock  is  followed  by  a very 

8 — 2 


ii6 


BIOMETRY 


gradual  return  towards  the  mean  character  of  the 
original  race.* 

It  must  be  remembered  that  in  the  calculation  which 
led  to  this  result  perfect  normal  variabihty  was  assumed, 
and  the  contribution  of  every  ancestor  of  the  same 
degree  to  the  hereditary  endowment  of  the  offspring 
was  supposed  to  be  exactly  equal.  Since  both  these 
assumptions  are  very  unlikely  to  be  realized  in  any 
actual  case,  the  statement  here  given  must  only  be 
regarded  as  an  approximate  indication  of  what  is 
likely  to  take  place. 

Some  remarkable  observations  have  been  published 
by  Professor  Johannsen,  of  Copenhagen,  and  from 
them  are  drawn  conclusions  which  seem  likely  to  lead 
to  a distinct  advance  in  our  understanding  of  the  pro- 
cess of  so-called  continuous  variation,  and  of  the  way 
in  which  such  variations  are  transmitted.  Johannsen’s 
conclusions  have  more  recently  received  remarkable 
confirmation  from  the  work  of  Jennings  on  protozoa, 
so  that  we  are  now  justified  in  accepting  the  theory 
of  the  pure  line  as  a well-established  working  hypo- 
thesis. We  shall  confine  our  present  account  to 
Johannsen’s  now  classical  observations. 

The  experiments  in  question  were  made  upon  plants 
which  could  be  self-fertilized  for  a series  of  generations. 
In  this  way  many  complications  were  avoided  which 
are  inevitably  introduced  in  the  case  of  biparental 

* From  this  it  seems  necessajrily  to  follow  that  it  is  impos- 
sible to  establish  a permanent  breed  simply  by  a process  of 
selection.  Professor  Pearson,  however,  avoids  tMs  conclusion. 


THEORY  OF  THE  PURE  LINE 


117 

inheritance.  Barley  and  kidney  beans  were  among 
the  plants  examined,  and  the  simplest  character  con- 
sidered was  the  size  of  the  seeds  of  the  latter  as 
measured  by  weighing.  In  this  particular  series  of 
experiments  each  plant  was  regarded  as  being 
characterized  by  the  average  weight  of  the  seeds 
which  it  produced. 

AU  the  descendants  arising  from  a single  plant  by  self- 
fertilization  are  spoken  of  by  J ohannsen  as  making  up 
a ‘ pure  line.’  And  the  members  of  such  a line  showed, 
in  respect  of  the  weight  of  their  seeds,  normal  varia- 
bility about  a mean  or  type  value.  The  general 
population  of  bean  plants,  made  up  of  a great  number 
of  such  pure  lines,  also  exhibited  a normal  curve  when 
the  weights  of  the  seeds  were  plotted.  The  pure 
lines  composing  such  a population  showed  various 
types,  some  of  them  elose  to  the  modal  value  of  the 
population,  but  others  differing  widely  from  it.  If 
now  a somewhat  widely  deviating  member  of  a par- 
ticular line  was  selected  for  propagation,  its  off- 
spring showed  regression  to  the  t3^pe  of  this  par- 
ticular line,  and  not  to  the  mean  value  of  the  general 
population. 

The  case  is  indeed  precisely  similar  to  the  supposed 
example  of  a mixture  of  races  of  peas,  which  was  made 
use  of  as  an  illustration  at  the  beginning  of  the  present 
chapter.  In  other  words,{_a  pure  line  consists  of  a 
group  of  individuals  which  has  a normal  variability  of 
its  own,  and  the  offspring  of  which  by  self-fertilization 
breed  true  to  the  type  of  their  own  particular  group. 


ii8 


BIOMETRY 


and  show  no  regression  towards  the  type  of  the  general 
population  to  which  the  group  belongs. 

If  we  were  to  carry  on  this  conception  to  the  case  of 
bisexual  inheritance,  we  should  find  that  the  different 
pure  lines  would  become  crossed  and  confused  together 
in  a way  which  would  be  very  difficult  to  disentangle. 
There  is  no  reason  to  doubt  that  statistical  treatment 
of  such  a population  would  yield  similar  results  to 
those  actually  obtained  by  biometricians  from  the 
data  at  their  disposal ; and  we  may  notice  that  a for- 
tuitous mixture  of  a considerable  number  of  pure 
lines,  having  shghtly  different  types,  would  admirably 
fulfil  the  conditions  we  have  seen  to  be  necessary  in 
the  case  of  material,  to  which  methods  based  upon 
the  theory  of  chance  are  to  be  applied.  The  phe- 
nomena which  follow  upon  the  crossing  together  of 
two  or  more  pure  lines  have  been  found,  in  the  majority 
of  cases  so  far  studied,  to  conform  to  those  laws  of 
heredity  associated  with  the  name  of  Mendel  which 
are  explained  in  Chapter  VII.  This  being  the  case, 
there  appears  to  be  every  probability  that  the  theory 
of  pure  lines,  in  combination  with  the  method  of  in- 
heritance referred  to,  may  adequately  serve  to  describe 
those  phenomena  to  account  for  which  the  law  of 
ancestral  inheritance  was  called  into  existence. 

The  conclusions  to  which  Professor  Johannsen’s 
experiments  lead  him  may  be  summed  up  as  follows  : 
Individuals  which  differ  (in  size,  for  example)  from 
the  mean  of  a population  give  rise  to  offspring  which 
differ  from  that  mean  value  in  the  same  direction 
but  to  a smaller  extent.  Selection,  therefore,  will 


THEORY  OF  THE  PURE  LINE 


119 

produce  a change  in  the  average  character  of  a popula- 
tion taken  as  a whole.  Selection  within  a pure  line 
produces  no  effect  of  this  kind.  The  average  character 
of  the  offspring  of  typical  members  of  the  hne  is  the 
same  as  that  of  the  offspring  of  members  which  show 
the  widest  deviations  from  the  type. 

Selection  in  a population  consists  in  the  partial 
separation  of  those  lines  the  types  of  which  differ  in 
the  required  direction  from  the  average  character  of 
the  population.  This  effect  must  of  necessity  come  to 
an  end  when  the  most  eccentric  line  is  completely 
isolated.  The  great  comphcations  introduced  when 
the  lines  are  intermingled  through  mixed  breeding  may 
make  this  process  of  isolation  a very  tedious  one. 

It  will  be  seen  that  the  values  calculated  by  Pearson 
to  represent  the  result  of  selection  in  a population 
agree  quite  well  with  Johannsen’s  explanation  of  the 
constitution  of  such  a population  out  of  a number 
of  pure  lines.  The  result  of  Professor  Johannsen’s 
further  experiments  will  therefore  be  awaited  with 
great  interest  by  biologists  and  biometricians  alike. 

On  the  theory  of  pure  Knes  it  is  to  be  noticed  that 
the  personal  character  of  a particular  ancestor  has 
no  influence  upon  his  descendants ; it  is  only  the 
type  of  the  line  to  which  he  belongs  which  influences 
the  offspring,  so  that  this  theory  is  in  perfect  agreement 
with  Weismann’s  theory  of  inheritance  as  described 
on  p.  74. 

It  is  also  to  be  observed  that  the  principle  of  the 
pure  line  appUes  only  to  quantitative  characters — 
such  characters  of  size,  or  of  weight,  or  of  proportion. 


120 


BIOMETRY 


as  are  very  seldom  made  use  of  by  systematists  for 
the  distinction  of  natural  species. 

BIBLIOGRAPHY. 

Galton,  F.  : Natural  Inheritance,  1889. 

Pearson,  K.  : The  Grammar  of  Science,  second  edition,  1900. 
Vernon,  H.  M.  : Variation  in  Animals  and  Plants,  1903. 
Davenport,  C.  B.  : Statistical  Methods,  1904. 

Biometrika,  1901 — 

JoHANNSEN,  W.  I Ueber  Erblichkeit  in  Populationen  und  in 
reinen  Linien,  1903. 

,,  Elemente  der  Exakten  Erbhchkeitslehre, 

1909. 


THE  LIBRARY 
OF  THE 

UfJIVERSlTY  OF  ILLINOIS 


Hugo  de  Vries. 


\To  Jace  p.  121. 


CHAPTER  V 


THE  THEORY  OF  MUTATION 

Mutation  is  the  term  applied  by  de  Vries  to  express 
the  process  of  origination  of  a new  species,  or  of  a new 
specific  character,  when  this  takes  place  by  the  dis- 
continuous method  at  a single  step — a process  which 
he  regards  as  the  most  important  if  not  the  sole 
method  by  which  new  species  or  specific  characters 
arise.  We  shall  see  that  although  de  Vries  has 
recently  done  much  to  forward  the  propagation  of 
this  idea,  the  belief  that  such  a discontinuous  process 
is  the  normal  method  by  which  new  species  come  into 
existence  has  been  developing  for  a considerable  time. 

We  have  seen  that  those  who  accept  the  idea  of 
evolution  by  the  action  of  natural  selection  upon  a 
series  of  minute  and  almost  imperceptible  variations 
are  confronted  with  the  difficulty  of  explaining  how 
by  this  method  there  could  arise  a number  of  different 
structures  or  parts  so  co-ordinated  as  to  share  in  a 
common  function.  Moreover,  a closer  examination 
of  the  actual  processes  of  variation  and  inheritance 
render  it  doubtful  whether  the  selection  of  continuous 
variations  of  even  a simple  characteristic  can  ever  lead 
to  the  development  of  a permanent  new  race.  The 

I2I 


X22 


THE  THEORY  OF  MUTATION 


result  of  Pearson’s  calculations,  described  in  the 
preceding  chapter,  seems  to  indicate  that  the  selection 
of  a certain  value  of  a particular  character  for  many 
generations  will  never  lead  to  the  formation  of  a race 
in  which  the  mean  value  of  the  character  is  as  high 
as  the  selected  value.  But,  says  the  selectionist,  it 
will  happen  in  Nat^re  that  as  the  standard  of  the 
race  is  raised  by  selection,  the  value  selected  will  be 
still  further  raised,  and  so  on,  and  in  this  way  an 
indefinite  amount  of  improvement  is  rendered  possible. 
If  Johannsen’s  conclusions  are  well  founded,  this  is 
clearly  not  the  case ; on  the  contrary,  there  is  a 
perfectly  definite  limit  to  the  effect  which  selection 
can  produce. 

The  question  whether  or  not  a gradual  method  of 
evolution  is  possible  has  not  yet  been  absolutely 
decided  for  any  single  species  or  character,  but  it  cer- 
tainly seems  that  now  for  the  first  time  the  possibility 
of  a definite  decision  is  within  sight.  At  the  same 
time  it  is  impossible  to  prove  a universal  negative.  If 
we  look  at  the  other  side  of  the  problem  we  shall  find 
that  the  evidence  in  favour  of  an  alternative  process 
has  multiplied  even  faster  than  the  evidence  against 
the  continuous  accumulation  of  minute  differences ; 
and  the  present  tendency  is  certainly  to  look  for  other 
sources  of  specific  distinctness  than  that  which  is 
offered  by  the  natural  selection  of . continuous  varia- 
tions. 

Even  before  the  new  evidence  which  we  have  briefly 
outlined  was  available,  Herbert  Spencer  found  the 
difi&culties  in  the  way  of  accepting  the  purely 


COORDINATED  STRUCTURES 


123 


Darwinian  explanation  to  be  so  great,  that  he  adopted 
the  hypothesis  of  the  inheritance  of  acquired  char- 
acters, as  being  the  only  adequate  explanation  of  the 
phenomena  which  was  in  his  time  available. 

Unfortunately,  satisfactory  evidence  that  such  a 
form  of  inheritance  ever  actually  takes  place  has  never 
been  forthcoming  in  sufficient  amount  to  lead  to 
universal  conviction.  Indeed,  at  the  present  day  the 
consensus  of  opinion  among  experts  is  undoubtedly 
to  the  effect  that  acquired  characters  are  not  inherited 
at  all,  except  in  so  far  as  better  nutrition  of  the  parent 
may  lead  to  the  production  of  more  vigorous  off- 
spring. And  it  seems  clear  that  such  an  effect  as  the 
latter  cannot  go  on  accumulating  for  more  than  a few 
generations. 

Thus  we  see  that  in  the  purely  Darwinian  view  there  is 
something  wanting,  whilst  the  Lamarckian  explanation 
is  ruled  out  of  court  for  the  present  for  lack  of  direct 
evidence.  If,  at  this  point,  we  find  that  in  Nature  a 
co-ordinated  set  of  structures  can  and  does  arise  in 
an  already  perfected  condition  at  a single  step,  and 
that  such  phenomena  take  place  with  sufficient  fre- 
quency to  give  ample  opportunities  for  the  survival 
of  the  new  type  so  arising,  we  have  at  once  discovered 
an  alternative  way  out  of  the  difficulty.  Such  a 
discovery  must  throw  abundant  light  on  the  obscurity 
overshadowing  the  methods  by  which  evolution  has 
taken  place,  even  though  we  may  not  yet  have  arrived 
at  any  kind  of  explanation  of  the  cause  of  this  phe- 
nomenon of  co-ordinated  and  definite  variability. 

The  actual  observation  of  variations  of  this  kind  is 


124 


THE  THEORY  OF  MUTATION 


of  quite  recent  date,  and  their  recognition  is  largely 
due  to  the  exertions  of  Bateson.  But  the  idea  that 
this  is  the  way  in  which  evolution  takes  place  is  very 
ancient,  and  seems  to  have  been  guessed  at  by  Aristotle. 
Perhaps  the  earliest  use  of  the  actual  word  ‘ muta- 
tion ’ in  this  sense  is  to  be  found  in  ‘ Pseudodoxia 
Epidemica,'  by  Sir.  Thomas  Browne.  I quote  from 
Book  VI.,  chapter  x.,  ‘Of  the  Blackness  of  Negroes  ’* 
(second  edition,  1650)  : ' We  may  say  that  men  became 
black  in  the  same  manner  that  some  Foxes,  Squirrels, 
Lions,  first  turned  of  this  complection,  whereof  there 
are  a constant  sort  in  diverse  Countries ; that  some 
Chaughes  came  to  have  red  legges  and  bills,  that  Crows 
became  pyed ; All  which  mutations,  however  they 
began,  depend  upon  durable  foundations,  and  such 
as  may  continue  for  ever.’ 

The  experiments  upon  cross-breeding,  which  are 
described  in  a later  chapter,  will  be  found  fully  to  bear 
out  the  idea  that  ‘ mutations,’  or  definite  character- 
istics which  have  arisen  in  a definite  way,  do  depend 
upon  durable  foundations. 

The  late  Professor  Huxley’s  emphatic  approval  of 
the  ‘ Origin  of  Species,’  as  signalized  in  his  reviews  of 
the  first  edition  of  that  work,  was  tempered  by  the 
following  mild  criticism : ‘ Mr.  Darwin’s  position 
might,  we  think,  have  been  even  stronger  than  it  is 
if  he  had  not  embarrassed  himself  with  the  aphorism 
“Natura  non  facit  saltum,”  which  turns  up  so  often 
in  his  pages.  We  believe  . . . that  Nature  does  make 
jumps  now  and  then,  and  a recognition  of  the  fact  is 

* I am  Indebted  to  my  friend  Mr.  R.  C.  Punnett  for  this 
reference. 


ORGANIC  STABILITY 


125 


of  no  small  importance  in  disposing  of  many  minor 
objections  to  the  doctrine  of  transmutation.’* 

The  first  person  to  formulate  a more  or  less  precise 
view  upon  the  subject  of  definite  variation  was  Francis 
Galton,  although  this  author  never  entered  into  the 
question  at  any  great  length.  Galton’s  attitude 
towards  the  problem  in  its  early  stages  may  be  gathered 
from  the  following  quotation  from  his  ‘ Natural  In- 
heritance ‘ The  theory  of  natural  selection  might 
dispense  with  a restriction  for  which  it  is  difficult  to 
see  either  the  need  or  the  justification — namely,  that 
the  course  of  evolution  always  proceeds  by  steps  that 
are  severally  minute,  and  that  become  effective  only 
through  accumulation.  That  the  steps  may  be  small, 
and  that  they  must  be  small,  are  very  different  views  ; 
it  is  only  to  the  latter  that  I object,  and  only  when  the 
indefinite  word  “ small  ” is  used  in  the  sense  of  “ barely 
discernible,”  or  as  small  compared  with  such  large  sports 
as  are  known  to  have  been  the  origins  of  new  races. ’f 
But  more  than  this,  the  idea  of  the  existence  of 
stable  forms,  such  as  may  be  supposed  to  have  arisen 
by  large  and  sudden  variations,  is  very  well  expressed 
by  Galton  in  his  division  of  varieties  into  the  three 
groups  of  primary  types,  subordinate  types,  and  mere 
deviations  from  the  latter.  A most  luminous  analogy 
is  affprded  by  the  three  t3q)es  of  public  vehicles  which 
at  the  end  of  the  nineteenth  century  were  character- 
istic of  the  streets  of  London  ; and  it  is  impossible  to 
resist  quoting  Galton’s  account  of  them.  These  three 

* ‘ Collected  Essays,’  vol.  ii.,  p.  77. 

t ‘ Natural  Inheritance,’  p.  3a 


126 


THE  THEORY  OF  MUTATION 


kinds  of  carriages,  ‘ namely,  omnibuses,  hansoms, 
and  four-wheelers,  are  specific  and  excellent  illustra- 
tions of  what  I wish  to  express  by  mechanical  types 
as  distinguished  from  subtypes.  Attempted  im- 
provements in  each  of  them  are  yearly  seen,  but  none 
have  as  yet  superseded  the  old  familiar  patterns, 
which  cannot,  as  it  thus  far  appears,  be  changed  with 
advantage,  taking  the  circumstances  of  London  as 
they  are.  Yet  there  have  been  numerous  subsidiary 
and  patented  contrivances,  each  a distinct  step  in 
the  improvement  of  one  or  other  of  the  three  primary 


types,  and  there  are  or  may  be  an  indefinite  number  of 
varieties  in  details,  too  unimportant  to  be  subjects  of 
patent  rights.’* 

More  recently  Galton  might  have  pointed  out  the 
introduction  of  motor  traffic  as  illustrating  a distinct 
mutation. 

The  distinction  between  primary  and  subordinate 
positions  of  stability  is  further  excellently  illustrated 
by  the  model  which  is  here  represented,  and  which 
is  known  as  Galton’s  polygon  (Fig.  lo). 

♦ ‘Natural  Inheritance,’  p.  26. 


A PHYSICAL  ANALOGY 


lay 

The  first  position  of  the  model,  resting  upon  the  side 
A B,  may  be  taken  to  represent  the  condition  of  a 
type  or  stable  form.  A comparatively  small  push 
(variation)  will  lead  to  the  production  of  the  subtype 
illustrated  by  the  position  B C.  When  in  this  new 
position,  it  is  easier  to  cause  the  model  to  return  to  its 
original  position  A B than  it  is  to  make  it  pass  on  to  the 
new  and  more  modified  position  resting  upon  the  side 
CD.  A strong  push  (mutation)  may  force  the  model 
to  pass  through  the  position  C D until  it  comes  to  rest  on 
the  side  opposite  to  A B.  This  fresh  position  represents 
a new  stable  form,  and  it  is  now  once  more  surrounded 
by  positions  of  subordinate  stability — subtypes. 

One  more  analogy  before  we  pass  on  to  consider  the 
more  recent  observations  upon  discontinuous  varia- 
tions or  mutations.  We  may  compare  the  difference 
which  exists  between  deviations  and  stable  forms, 
arising  by  fluctuating  and  by  definite  variation  respec- 
tively, with  the  behaviour  of  the  atoms  of  chemistry, 
as  expressed  in  the  account  of  their  structure  recently 
given  by  Professor  Sir  J.  J.  Thomson.  Such  an  atom 
is  regarded  as  being  made  up  of  a number  of  electrons 
or  corpuscles  bearing  definite  relations  to  one  another 
in  space.  In  certain  circumstances  it  seems  that  it 
may  be  possible  to  remove  a series  of  these  corpuscles 
from  the  atom  one  at  a time,  in  which  case  every  such 
successive  removal  would  be  accompanied  by  a com- 
paratively gradual  and  progressive  change  in  the 
properties  of  the  atom  so  modified.  But  after  a 
certain  time  a point  would  be  reached  at  which  the 
removal  of  one  more  electron  would  necessitate  a 


128 


THE  THEORY  OF  MUTATION 


complete  rearrangement  of  the  remaining  corpuscles 
in  order  to  arrive  at  a new  position  of  equilibrium, 
and  this  change  would  be  accompanied  by  a marked 
alteration  in  the  chemical  properties  of  the  atom  itself. 
In  like  manner  the  chemical  composition  of  the  living 
substance  of  a race  of  organisms  may  be  conceived  to 
alter  step  by  step,  every  such  step  being  accompanied 
by  comparatively  unimportant  changes  in  its  visible 
characters,  until  the  time  arrives  when  any  further 
alteration  must  be  associated  with  a deep-seated  revo- 
lution in  the  constitution  of  the  living  substance,  and 
with  a corresponding  marked  mutation  in  the  external 
features  of  the  members  of  the  race. 

The  first  really  definite  attempt  to  collect  and  co- 
ordinate the  facts  of  discontinuous  variation  was  made 
by  Bateson  in  his  book  entitled  ‘ Materials  for  the 
Study  of  Variation,’  published  in  1894.  The  intro- 
duction and  concluding  remarks  at  least  of  this  volume 
ought  to  be  read  by  everyone  who  is  interested  in 
these  subjects.  The  bulk  of  the  book  contains  a mass 
of  material  of  great  value  to  specialists. 

After  pointing  out  the  difiiculties  which  prevent  his 
acceptance  of  the  orthodox  belief  in  the  origin  of  dis- 
continuous and  apparently  adaptative  types  of  animals 
and  plants  through  the  action  of  natural  selection  on 
minute  variations,  difficulties  to  which  we  have 
already  paid  some  attention,  Bateson  records  his  con- 
viction that  the  facts  of  discontinuous  variation  afford 
a way  out  of  the  difficulty.  He  shows  (i)  that  differ- 
ences of  the  kind  which  are  generally  used  to  dis- 
tinguish separate  species  may  arise  as  single  variations  ; 


SYMMETRY 


129 


(2)  that  such  a form  of  variation  is  by  no  means  so 
uncommon  a phenomenon  as  was  formerly  supposed  ; 
and  (3)  that  variations  of  this  kind  may  occur  in 
every  description  of  organ  and  part  in  a number  of 
different  plants  and  animals.  The  facts  with  which 
the  main  bulk  of  the  book  is  concerned  have  reference 
to  the  animal  kingdom. 

We  shall  find  it  profitable  to  consider  the  views 
expressed  in  this  book  a little  more  closely,  though  it 
would  occupy  too  much  space  to  give  even  a brief 
summary  of  the  facts  upon  which  they  are  based,  and 
for  which  reference  must  be  made  to  the  original. 

In  the  first  place  Bateson  calls  attention  to  the 
phenomenon  of  symmetry  as  being  a characteristic 
feature  common  to  almost  all  organisms.  This  sym- 
metry may  manifest  itself  in  a number  of  different 
ways.  In  bilateral  and  radial  symmetry  the  parts 
symmetrically  disposed  are  related  to  one  another  in 
the  same  kind  of  way  as  are  an  object  and  its  image 
reflected  in  a plane  mirror.  Such  symmetry,  as, 
indeed,  every  kind  of  symmetry,  is  usually  associated 
with  a repetition  of  parts.  In  the  present  instances 
the  parts  are  repeated  in  pairs,  as  with  the  two  eyes 
in  the  human  face  ; or  in  a radial  series,  like  the  arms 
of  a star-fish,  or  the  petals  of  a buttercup.  To  this 
phenomenon  of  the  repetition  of  parts,  generally 
occurring  in  such  a way  as  to  produce  a symmetry  or 
pattern,  the  term  merism  is  applied. 

Symmetry  may  affect  the  proportions  and  shape 
of  the  body  of  an  animal  or  plant  as  a whole,  or,  on 
the  other  hand,  separate  parts  or  organs  may  show  a 

9 


130 


THE  THEORY  OF  MUTATION 


separate  symmetry  of  their  own.  For  the  phenomena 
thus  distinguishable  separate  terms  are  proposed.  A 
major  symmetry  is  a form  of  pattern  which  includes  the 
body  as  a whole,  as  in  the  case  of  most  animals  where 
the  two  sides  of  the  body  closely  resemble  one  another. 
A minor  symmetry  is  a pattern  completed  in  a separate 
organ  or  part — for  instance,  in  the  flower  of  a plant  or 
the  limb  of  an  animal. 

Once  more  we  may  lay  stress  upon  the  universal 
existence  of  pattern  among  living  things.  Bateson 
points  out  that  in  collecting  any  kind  of  living  creature 
it  is  the  symmetry  of  it  which,  as  a general  rule,  first 
catches  the  eye  and  distinguishes  the  organized  body 
from  surrounding  inanimate  objects. 

The  phenomenon  of  merism  or  repetition  of  parts 
being  understood,  we  are  in  a position  to  consider  the 
subdivision  of  variations  into  meristic  variations  and 
substantive  variations  respectively. 

Meristic  variations  are  variations  in  symmetry  and 
in  the  number  of  repeated  parts.  A change  in  the 
number  of  organs  in  a series  may  conceivably  take 
place  gradually  by  the  addition  or  subtraction  of  suc- 
cessive fractions  of  a part.  But,  as  a matter  of  fact, 
this  is  very  seldom  the  case.  The  increase  or  decrease 
usually  involves  one  whole  member  at  a time  and  some- 
times more,  so  that  this  kind  of  variation  is,  as  a rule, 
discontinuous.  Abundant  illustrations  of  this  fact  are 
to  be  found  in  the  case  of  changes  in  the  number  of 
such  parts  as  the  teeth  or  vertebrae  of  mammals  ; and 
a particulaily  good  instance  is  afforded  by  the  variations 
which  take  place  in  the  number  of  ray  florets  in  various 


MERISTIC  VARIATIONS 


131 


composite  plants — e.g.,  the  daisy  and  chrysanthemum. 
It  is  suggested  that  meristic  variations  are  connected 
with  definite  changes  in  the  mechanical  relations  of 
dividing  parts,  and  that  it  is  in  the  mechanics  of  cell- 
division  that  the  explanation  of  their  discontinuous 
appearance  is  to  be  sought  for. 

Thus  when,  for  example,  a tulip-flower  appears 
having  its  parts  perfectly  developed  in  sets  of  four 
instead  of  in  sets  of  three,  it  is  suggested  that  the 
arrangement  in  fours,  like  the  arrangement  in  threes, 
fulfils  certain  conditions  of  equilibrium  among  the 
forces  which  affect  the  cell-divisions  in  the  rudiment 
of  the  flower,  and  that  these  conditions  of  stability 
would  not  be  equally  well  provided  for  by  any  inter- 
mediate arrangement. 

Substantive  variations  are  changes  in  the  actual 
constitution  or  substance  of  the  parts  themselves. 
For  example,  a plant  with  coloured  flowers  may  give 
rise  to  offspring  the  flowers  of  which  are  white.  There 
seems  to  be  no  mechanical  necessity  for  such  varia- 
tions to  be  discontinuous  rather  than  continuous  ; it 
is  quite  possible  to  imagine  a gradual  dilution  of  colour 
taking  place  throughout  a long  series  of  generations. 
Discontinuous  substantive  variations  are,  however,  not 
infrequent,  and  in  such  cases  it  is  suggested  that  they 
may  be  associated  with  definite  changes  in  chemical 
composition.  Thus,  for  example,  definite  alterations 
in  the  colour  of  offspring  as  compared  with  their 
parents  are  almost  necessarily  of  a chemical  nature. 

The  further  evidence  contained  in  the  book  we  are 
considering  refers  entirely  to  meristic  variation. 

9—2 


132 


THE  THEORY  OF  MUTATION 


An  important  point  with  regard  to  repeated  parts 
is  to  be  observed  in  the  fact  that  in  a pair  of  allied 
species,  in  which  a series  of  repeated  organs  in  the  one 
is  clearly  comparable  with  a similar  series  in  the  other, 
aU  the  parts  in  one  form  may  differ  from  those  in  the 
second  by  the  same  kind  of  distinction,  whether  this 
be  qualitative  or  numerical.  The  facts  suggest  strongly 
that  such  cases  are  to  be  accounted  lor  by  all  the 
parts  in  question  in  one  or  both  species  having  varied 
in  a similar  way  at  the  same  time  rather  than  in  suc- 
cession. The  occurrence  of  such  a similar  and  simul- 
taneous process  of  variation  of  repeated  parts  clearly 
simplifies  in  a marked  degree  the  process  of  evolution, 
and  greatly  reduces  the  time  which  would  be  required 
for  this  process,  if  similar  changes  in  repeated  parts 
always  took  place  successively.  If  we  take  an  ex- 
treme case  the  latter  supposition  becomes  absurd.  In 
the  albino  or  pure  white  types  which  occur  as  varia- 
tions in  many  species  of  birds  and  mammals  it  is 
obvious  that  every  hair  or  feather  has  taken  on  the 
white  colour  at  the  same  time  and  for  the  same  reason, 
whatever  that  reason  may  have  been.  Hairs  or 
feathers  are  very  good  examples  of  repeated  parts  of 
the  kind  of  which  we  have  been  speaking.  It  appears, 
too,  that  colour  patterns  may  originate  and  change  in 
a similar  manner.  In  the  case  of  such  a bird  as  the 
peacock  we  should  expect  on  this  view  that  the  pattern 
varied  in  all  the  tail  feathers  simultaneously,  nor  is  it 
necessary  to  suppose  that  even  this  process  took  place 
by  a very  long  series  of  minute  steps.  If  we  find  that 
the  splendid  coloration  of  the  peacock’s  tail  arose 


HOMCEOSIS 


133 


by  a few  marked  variations,  each  of  which  occurred 
simultaneously  in  all  the  feathers  at  once,  several 
serious  difficulties  are  avoided,  and  on  the  analogy  of 
similar  known  cases  we  have  every  reason  to  believe 
that  this  was  so.  And  similar  changes  may  take  place 
in  cases  where  the  pattern  depends  on  the  coloration 
of  a group  of  feathers  or  hairs.  Indeed,  if  we  con- 
sider, we  shall  find  it  very  difficult  to  picture  such  a 
process  as  taking  place  in  any  other  way.  We  can 
scarcely  suppose  the  spots  of  the  leopard,  for  instance, 
to  have  arisen  one  at  a time. 

An  important  kind  of  discontinuous  variation  is 
that  to  which  Bateson  has  applied  the  term  homceosis. 
The  same  sort  of  change  had  previously  been  described 
by  Masters  in  the  case  of  plants  under  the  name 
‘ metamorphy,’  but  the  latter  expression  has  also 
been  employed  in  other  senses.  Homceosis  consists 
in  the  assumption  by  one  member  of  a meristic  series 
of  the  form  or  character  proper  to  another  member 
of  the  same  series  ; for  example,  the  modification  of 
the  petal  of  a flower  into  a stamen,  or  of  the  eye  of  a 
crab  into  an  antenna- like  organ. 

‘In  these  cases  a limb,  a floral  segment,, or  some 
other  member,  though  itself  a group  of  miscellaneous 
tissues,  may  suddenly  appear  in  the  likeness  of  some 
other  member  of  the  series,  assuming  at  one  step  the 
condition  to  which  the  member  copied  attained  pre- 
sumably by  a long  course  of  evolution.’  * 

The  phenomenon  of  homceosis  is  frequently  to  be 
seen  among  the  parts  of  flowers.  Double  flowers  in 
* ‘Materials  for  the  Study  of  Variation,’  p.  570. 


134 


THE  THEORY  OF  MUTATION 


many  cases — for  instance,  in  the  case  of  the  rose — arise 
by  the  development  of  petal-like  organs  in  the  position 
which  would  properly  be  occupied  by  stamens.  A 
parallel  process  is  to  be  seen  in  the  heads  of  com- 
posite flowers,  such  as  the  chrysanthemum.  In  a 
double  chrysanthemum  the  florets  of  the  disc  develop 
in  the  likeness  of  ray  florets.  Both  these  cases  would 
be  classed  as  examples  of  outward  homceosis,  because 
the  parts  concerned  resemble  organs  normally  de- 
veloped in  a whorl  exterior  to  themselves.  A case  of 
inward  homceosis,  on  the  other  hand,  is  afforded  by 
the  appearance  of  a petaloid  calyx — ^for  example,  in 
a tobacco-plant — the  outermost  whorl  of  the  flower 
taking  on  the  appearance  of  a whorl  internal  to  itself. 

In  cases  such  as  these  we  observe  once  more  the 
occurrence  of  a marked  and  definite  change,  which, 
though  at  first  sight  quite  distinct  from  the  method  of 
similar  and  simultaneous  variation,  yet  bears  a certain 
resemblance  to  that  process  in  the  fact  that  the  direc- 
tion in  which  a particular  part  varies  is  not  wholly 
unrelated  to  the  behaviour  of  other  parts  of  the  same 
organism.  The  process  thus,  briefly  described  seems 
likely  to  have  had  considerable  importance  in  evolu- 
tion, notably  in  the  origin  of  differences  in  the  numerical 
relations  of  the  bones  in  various  parts  of  the  spinal 
column  in  different  vertebrate  animals. 

The  preceding  account  of  the  conclusions  drawn 
from  Bateson’s  laborious  study  of  variation  has  in- 
volved a good  deal  of  technicality,  but  this  is,  un- 
fortunately, unavoidable.  The  point  chiefly  to  be 
emphasized  is  the  frequent  occurrence  in  Nature  of 


EFFECT  OF  SELECTION 


135 


variations  of  a definite  or  discontinuous  type — the 
fact  that  differences  of  the  kind  "which  are  constantly 
used  to  distinguish  natural  species  can  and  do  arise 
in  Nature  at  a single  step,  so  that  it  is  not  necessary 
for  such  differences  to  be  built  up  gradually  by  the 
action  of  natural  selection. 

De  Vries,  in  his  ‘ Mutations  Theorie,’  goes  further 
than  this,  and  attacks  the  position  held  by  those  who 
accept  the  doctrine  that  natural  selection  of  individual 
differences  can  ever  lead  to  definite  and  permanent,  or 
specific,  distinctions.  Indeed,  one  of  the  chief  contri- 
butions of  this  author  to  the  species  controversy  is  to 
point  out  that  the  belief  that  artificial  selection  acts 
in  this  way  upon  domestic  plants  is  based  upon  a mis- 
apprehension. De  Vries  himself  has  carried  out  a 
number  of  experiihents  in  selection,  and  he  comes  to 
the  conclusion  that  selection  of  ordinary  individual 
differences  has  no  permanent  effect  at  all.*  The  actual 
effect  of  this  kind  of  selection  is  well  illustrated  by  the 
results  of  the  processes  employed  in  the  sugar-beet 
industry,  in  which  elaborate  care  is  taken  to  select 
those  roots  which  contain  the  highest  percentage  of 
sugar  for  the  purpose  of  propagation.  This  process 
was  followed  at  first  by  a rapid  improvement,  but  the 
rate  at  which  the  percentage  of  sugar  increased  soon 
fell  off,  until  at  the  present  day  all  that  selection  can 
effect  is  to  keep  up  the  standard  of  excellence  already 
attained.  Moreover,  that  this  process  of  improvement 

* Compare,  however,  Johanssen’s  more  recent  conclusions 
see  p.  1 1 8). 


136  THE  THEORY  OF  MUTATION 


was  a very  gradual  one  is  to  be  accounted  for  in  part, 
at  least,  from  the  fact  that  the  methods  of  selection 
themselves  gradually  improved  from  year  to  year. 
There  is  no  reason  to  doubt  that  a thoroughly  efficient 
method  of  selection  would  have  worked  its  full  effect 
in  a few  generations.  A similar  state  of  things  is  said 
to  be  the  case  with  the  cereals,  such  as  wheat  and 
barley,  which  have  been  selected  largely  for  the  size 
of  the  grains.  From  his  own  experiments,  de  Vries 
has  come  to  the  conclusion  that,  when  selection  is 
really  efficient,  the  full  possible  effect  of  this  process 
is  exhausted  in  quite  a small  number  of  generations, 
and  that  then  the  only  further  effect  of  selection  is  to 
keep  up  the  standard  already  arrived  at. 

We  have  seen  that  the  theoretical  conclusions  of  the 
biometricians  are  in  agreement  with  the  opinions  here 
expressed,  so  long  as  selection  is  understood  to  be  con- 
fined to  the  choosing  out  of  parents  which  show  a 
definite  standard  value  of  the  character  under  con- 
sideration, this  value  being  the  same  in  each  genera- 
tion. Under  these  circumstances.  Professor  Pearson 
concludes  that  in  the  first  two  or  three  generations 
a marked  advance  in  the  desired  direction  will  take 
place,  but  that  further  selection  (in  this  sense)  will 
have  comparatively  little  effect.  But  the  believer 
in  continuous  evolution  maintains  in  addition  that 
selection  will  be  foHow'ed  to  an  indefinite  extent  by 
further  variations  in  the  direction  of  selection,  since 
otherwise  selection  could  never  lead  to  important 
changes  in  organization.  In  the  face  of  the  strong 
contrary  evidence,  and  of  the  fact  that  alternative 


EFFECT  OF  SELECTION 


137 


methods  of  evolution  are  now  known  to  be  available, 
the  burden  of  proof  of  this  proposition  seems  to 
lie  with  those  who  maintain  the  all-important  influ- 
ence of  continuous  variation  and  selection.  At  present 
we  are  free  to  reply  in  the  words  of  Malthus,  who  long 
ago  protested  against  the  extravagant  powers  which 
were  ascribed  to  the  selection  of  small  differences. 

‘ I have  been  told,’  Malthus  writes,  ‘ that  it  is  a 
maxim  among  some  of  the  improvers  of  cattle  that 
you  may  breed  to  any  degree  of  nicety  you  please, 
and  they  found  this  maxim  upon  another,  which  is, 
that  some  of  the  offspring  will  possess  the  desirable 
qualities  of  the  parents  in  a greater  degree.  In  the 
famous  Leicestershire  breed  of  sheep,  the  object  is  to 
procure  them  with  small  heads  and  small  legs.  Pro- 
ceeding upon  these  breeding  maxims,  it  is  evident  that 
we  might  go  on  until  the  heads  and  legs  were  evan- 
escent quantities  ; but  this  is  so  palpable  an  absurdity 
that  we  may  be  quite  sure  the  premises  are  not  just, 
and  that  there  really  is  a limit,  though  we  cannot  see 
it  or  say  exactly  where  it  is.’  * 

The  only  recorded  example  I am  aware  of  in  the 
case  of  animals,  which  shows  the  result  of  long-con- 
tinued selection  acting  upon  a quantitative  character, 
is  afforded  by  the  case  of  the  American  trotting-horse. 
In  this  case  it  appears  highly  probable  that  we  are 
dealing  with  a character  which  varies  in  a strictly 
continuous  fashion.  In  his  book  upon  ‘ The  Trotting 
and  Pacing  Horse  in  America,’  Hamilton  Busbey 
gives  a table  from  which  the  diagram  on  p.  138 
* ‘ Essay  on  Population,’  6th  ed.,  vol.  ii.,  p.  ii. 


138 


THE  THEORY  OF  MUTATION 


is  constructed.  The  entries  in  this  table  show  the 
fastest  times  recorded  for  the  feat  of  trotting  a measured 
mile  in  various  years  beginning  with  i8i8.  The  ver- 
tical scale  contains  the  times,  which  vary  from  three 
minutes  down  to  one  minute  fifty-six  seconds,  and  the 
horizontal  scale  shows  the  year  in  which  the  record  was 


2,00- 

2,10  - 

2.20 

2,30 

2,40 

2,60 

3.00 


Year  !C20  ‘30  ’4-0  ’so  '60  ’70  ’80  ’90  1900 

Fig.  II. 

The  figures  to  the  left  of  the  diagram  are  to  be  read  as  minutes  and 

seconds. 

made.  Some  part  of  the  improvement  shown  is  clearly 
to  be  associated  with  better  tracks,  improved  methods 
of  training,  etc.,  but  these  will  scarcely  affect  the 
general  character  of  the  improvement  due  to  selec- 
tion. As  may  readily  be  seen  from  the  diagram,  the 
improvement  is  at  first  rapid,  but  afterwards  becomes 


ar  ~!C20  ^30  ^0  ^0  '60  ’70  ’80  ’90  1900 


EFFECT  OF  SELECTION 


139 


gradually  slower  and  slower.  At  the  end  of  the  series 
two  sudden  steps  upward  break  the  general  regularity 
of  the  series  of  records.  But  on  examination  of  the 
evidence  it  is  found  that  these  are  associated  with 
special  conditions,  and  are  not  really  exceptional.  The 
first  of  these  breaks — that  which  occurs  in  1892 — is 
coincident  with  the  introduction  of  a new  type  of  sulky, 
having  ball  bearings  and  other  improvements ; whilst 
the  record  of  1903  was  accomplished  behind  a pace- 
maker carrying  a wind-shield.  Neither  of  these  records, 
therefore,  is  strictly  comparable  with  the  rest  of  the 
series. 

The  observations  in  this  case  do  not,  indeed,  seem 
to  be  sufiicient  to  afford  the  basis  for  a final  decision 
against  the  theory  of  the  indeterminate  power  of  selec- 
tion. Yet  Malthus’  criticism  clearly  applies  very 
definitely  to  such  a case — i.e.,  there  must  be  a limit 
beyond  which  the  speed  of  the  trotting-horse  will 
never  improve  without  a fundamental  change  taking 
place  in  his  organization.  It  seems,  therefore,  safe  to 
conclude  that  the  curve  to  which  the  series  of  records 
approaches  is  of  the  character  of  a parabola — i.e.,  one 
which  continually  becomes  more  and  more  nearly 
horizontal — as  the  speed  of  the  horse  gradually  ap- 
proaches its  highest  possible  limit. 

De  Vries,  then,  contends  that  all  new  domestic  breeds 
have  arisen  by  the  discontinuous  method  as  definite 
novelties.  Darwin  himself  was  perfectly  aware  that 
this  is  usually  the  case,  but  the  conclusion  which  he 
drew  from  the  fact  was  a different  one,  as  the  following 
passage  shows ; 


140 


THE  THEORY  OF  MUTATION 


‘ He  (man)  often  begins  his  selection  by  some  half- 
monstrous  form,  or  at  least  by  some  modification  pro- 
minent enough  to  catch  the  eye  or  to  be  plainly  useful 
to  him.’  But  he  goes  on  ; ‘ Under  Nature,  the  slightest 
differences  of  structure  or  constitution  may  well  turn 
the  nicely-balanced  scale  in  the  struggle  for  life,  and 
so  be  preserved.’  * 

Of  the  origin  of  a new  type  of  plant  in  this  definite 
and  sudden  fashion,  the  Shirley  poppies  afford  an 
excellent  example.  These  originated  in  a mutation 
of  the  common  wild  field-poppy  {Pap aver  rhceas).  In 
1880  the  Rev.  W.  Wilks,  Vicar  of  Shirley,  near  Croydon, 
noticed  among  a patch  of  this  plant  growing  in  a waste 
corner  of  his  garden  a solitary  flower,  the  petals  of 
which  showed  a very  narrow  border  of  white.  The 
seeds  which  this  flower  produced  were  sown,  and  next 
year,  out  of  about  two  hundred  plants,  there  were  four 
or  five  upon  which  all  the  flowers  showed  the  same 
modification.  From  these,  by  further  horticultural 
processes,  the  strain  of  Shirley  poppies  originated. 
We  may  point  out  in  passing  that  if  the  original  plant 
had  been  self-pollinated,  a much  larger  proportion  of 
the  new  type  might  have  been  expected  to  appear  in 
the  next  generation. 

In  the  course  of  his  own  experiments,  de  Vries  has 
obtained  quite  a number  of  new  types  of  plants  by 
methods  like  the  above.  It  is  to  be  observed  that  the 
novelty  in  these  cases  usually  shows  a considerable 
range  of  normal  variability  of  its  own,  and  that  its 
first  appearance  is  generally  in  the  form  of  an  extreme 
♦ ‘ Origin  of  Species,’  6th.  ed.,  p.  60. 


EFFECT  OF  SELECTION 


141 

negative  variation*  from  the  proper  type  of  the  new 
variety.  In  this  way  the  novelty  may  not  appear  to  be 
very  far  removed  from  a high  normal  variation  of  the 
original  type.  The  behaviour  of  the  progeny  of  the  two 
types,  however — types  which  might  thus  in  themselves 
be  readily  confused — is  entirely  different,  and  a ready 
means  of  distinguishing  them  is  thereby  provided. 
Each  set  of  offspring  shows  regression  to  its  own  proper 
modal  value ; so  that  the  offspring  of  the  novelty  show 
a further  marked  development  of  the  new  features, 
whilst  the  offspring  of  an  extreme  normal  variation 
resemble  the  type  of  the  original  form  more  closely 
than  they  do  their  own  immediate  progenitor. 

If  new  types  are  not  produced  among  domesticated 
productions  by  the  action  of  artificial  selection,  and  all 
that  selection  can  effect  is  to  pick  out  definite  novelties 
when  they  occur,  the  analogy  between  natural  selec- 
tion and  artificial  selection  breaks  down,  and  a large 
and  important  section  of  the  evidence  in  favour  of  the 
production  of  natural  species  by  the  action  of  natural 
selection  is  destroyed.  In  the  place  of  this  explana- 
tion dearies  would  put  the  theory  of  mutation,  ac- 
cording to  which  new  species  arise  by  single  steps  as 
definite  novelties,  just  in  the  same  way  as  we  find  that 
domestic  varieties  are  produced.  More  than  this,  de 
Vries  believes  that  he  has  discovered  a set  of  new 
species  in  the  very  act  of  originating  from  an  old 
one  in  this  way,  a discovery  which  affords  the  basis 

* I.e.,  a variant  belonging  to  a class  situated  some  dis- 
tance from  the  mode  of  normal  variability  of  the  novelty, 
and  on  the  side  of  it  nearest  to  the  mode  of  the  original  type. 


142 


THE  THEORY  OF  MUTATION 


and  groundwork  of  the  views  which  he  puts  for- 
ward. 

The  plant  which  afforded  the  material  for  this  dis- 
covery is  known  as  (Enothera  Lamarckiana — that  is  to 
say,  this  is  the  name  of  the  old  species  from  which  the 
new  species  were  found  to  be  arising.  O.  Lamarckiana 
is  an  American  plant,  but  the  specimens  which  de  Vries 
found  to  be  in  a state  of  mutation  had  made  their 
escape  from  a garden,  and  were  running  wild  over  a 
disused  potato-field  near  a town  called  Hilversum,  in 
Holland.  On  examining  these  plants,  de  Vries  found 
two  distinct  new  forms,  which  were  quite  unlike  the 
remainder.  Each  kind  occurred  in  an  isolated  patch, 
as  if  it  had  arisen  from  the  seed  of  a single  plant. 

No  description  of  either  of  these  forms  was  to  be 
found  in  botanical  literature,  nor  were  there  specimens 
of  them  in  any  of  the  great  herbaria.  But  when  de 
Vries  took  seeds  from  some  of  the  plants  and  sowed 
them  in  his  garden,  he  found  that  the  new  forms  came 
true  to  type  — the  plants  produced  resembled  the 
parents  from  which  the  seeds  were  taken,  and  not  the 
normal  form  of  0.  Lamarckiana. 

Here,  then,  we  have  a case  in  which  two  new  species 
had  originated  from  an  old  one  in  a state  of  nature. 
But  de  Vries  went  further  than  this,  and  took  measures 
for  observing  the  actual  origin  of  new  forms  in  the 
cultivated  offspring  of  the  semi-wild  (Enothera. 

For  this  purpose  he  transplanted  a number  of  roots 
from  the  field  where  they  were  growing,  and  also  took 
seed  from  a number  of  other  plants,  and  from  these  he 
cultivated  large  numbers  of  seedlings  for  a series  of 


THE  LIBRARY 
OF  THE 

UNIVERSITY  OF  ILLINOIS 


Fig.  12. — Mutation  in  CEnothera. 


{From  de  Vries.) 


Top  row  . . 

..  Lam. 

lata 

Lam, 

Second  row 

. . siihovata 

alhida 

Lam. 

Third  row  . . 

, . alhida 

albtda 

lata 

Fourth  row 

. , oblonga 

lata 

Lam. 

Fifth  row  . . 

. . Lam. 

Lain. 

nibrinevvis. 

[To  face  p»  143. 


MUTATION  IN  (ENOTHERA 


143 


generations.  The  net  result  of  his  experiments  was 
this  : out  of  about  50,000  individuals  which  were  grown 
to  a recognisable  stage,  more  than  800  showed  muta- 
tion— that  is  to  say,  they  differed  specifically  from 
the  parent  0.  Lamarckiana.  The  800  individuals 
belonged  to  about  fifteen  new  kinds,  most  of  which 
appeared  repeatedly,  though  some  were  more  frequent 
than  others.  The  process  of  mutation  had,  therefore, 
taken  place  in  about  per  cent,  of  the  seedlings  which 
were  grown,  and  owing  to  various  reasons  this  estimate 
is  probably  considerably  too  low.  For  example,  many 
of  the  new  forms  were  very  weakly,  and  often  died 
before  it  was  possible  to  distinguish  them.  Others, 
again,  could  not  be  recognised  until  an  advanced  stage 
of  their  growth  had  been  reached,  whereas  only  a small 
proportion  of  the  seedlings  raised  could  be  grown  after 
they  had  reached  any  considerable  size,  owing  to  con- 
siderations of  space. 

We  cannot  now  follow  de  Vries  veiy  far  into  his 
elaborate  account  of  his  new  species  and  of  the  way 
in  which  they  originated  ; a few  general  remarks  only 
must  suffice.  Many  of  the  new  forms  were  recognis- 
able as  quite  young  seedlings,  notably  0.  albida, 
others  not  until  a much  later  period  of  their  growth. 
0.  gigas  was  the  finest  and  strongest  of  the  new  forms, 
but  only  made  its  appearance  on  two  occasions. 
0.  lata  also  appeared  to  be  as  strong  as  the  parental 
type,  whilst  two  other  forms  were  able  to  survive  in 
nature  in  competition  with  the  original  species,  as 
has  been  already  described.  Other  forms  which  were 
grown  and  flowered  were  plainly  less  well  fitted  for 


144 


THE  THEORY  OF  MUTATION 


the  battle  of  life  than  0.  Lamarckiana,  and  only  reached 
the  flowering  stage  by  the  help  of  careful  cultivation, 
and  others,  again,  were  never  got  to  flower  at  all. 
Some  of  the  latter,  however,  were  readily  distinguish- 
able by  the  strikingly  original  types  of  radical  leaves 
which  they  exhibited. 

When  they  had  once  made  their  appearance,  the 
majority  of  the  new  tji'pes  came  true  to  seed.  Some- 
times new  mutations  appeared  among  their  offspring, 
but  these  always  appeared  in  smaller  numbers  than 
among  the  offspring  of  the  parent  0.  Lamarckiana,  and 
some  of  the  commoner  mutations  were  usually  omitted, 
so  that  it  appeared  as  if  the  process  of  mutation  was 
accompanied  by  a tendency  towards  a fresh  stability. 
Some  of  the  most  marked  new  forms  came  quite  true 
so  far  as  the  observations  were  carried. 

Speaking  generally,  the  nature  of  the  differences 
which  distinguished  the  new  forms  from  the  parental 
species  was  just  of  the  same  type  as  that  of  those  which 
distinguish  Jordan’s  species  when  found  in  nature. 
The  differences  were  not,  as  a rule,  of  the  sort  shown 
when  new  garden  varieties  arise  as  sports.  An  example 
of  this  latter  kind  occurred,  however,  in  the  case  of  the 
new  form  0.  nanella,  which  was  a dwarf  or  permanently 
stunted  form,  but  in  other  respects  closely  resembled 
the  parent  type.  Apart  from  this,  the  new  forms 
appeared  to  be  given  off  quite  at  random,  without 
showing  any  definite  tendency  towards  progress  in  a 
particular  direction.  One  of  the  new  species  was 
almost  sterile  as  far  as  its  ovules  were  concerned, 
though  producing  good  pollen,  whilst  in  another  the 


MUTATION  IN  (ENOTHERA 


T45 

formation  of  the  pollen  was  very  defective.  None  of 
the  others  was  lacking  in  either  of  these  respects.  Each 
new  form  was  distinguished  by  certain  definite  features 
which  affected  almost  all  its  parts,  not  by  one  new 
character  only ; and  these  features  were  never  separable, 
but  always  appeared  in  common  on  the  same  plant. 

The  new  species,  of  course,  showed  normal  fluctuat- 
ing variability,  and,  as  an  extreme  result  of  this  varia- 
bility, forms  occasionally  appeared  midway  between 
one  of  the  new  species  and  the  parental  type.  In  such 
cases,  when  the  self-fertilized  seed  of  the  plant  showing 
such  an  intermediate  character  was  sown,  the  offspring 
were  found  to  group  themselves  round  the  normal 
form  of  the  new  species  or  round  that  of  the  parent 
Lamarckiana,  thus  affording  evidence  as  to  the  true 
nature  of  their  parent. 

Whether  or  not  we  are  prepared  to  accept  the  whole 
of  de  Vries’  conclusions  from  his  experiments,  we  can 
see  at  least  that  from  one  point  of  view  they  are  of  the 
very  greatest  importance.  For  before  de  Vries  pub- 
lished this  work  it  had  been  supposed  to  be  quite  im- 
possible to  make  direct  observations  upon  the  manner 
of  origin  of  new  species  in  Nature.  De  Vries  has  now 
shown  that  such  observations  can  be  made,  and  this  is 
in  itself  a most  valuable  piece  of  information.  He  has 
introduced  an  entirely  new  method  into  the  domain  of 
species  research,  and  one  by  the  use  of  which  it  is  to  be 
hoped  that  before  long  a definite  answer  will  be  obtained 
to  the  question  whether  species  in  general  arise  by  defi- 
nite steps,  or  with  an  imperceptible  degree  of  slowness. 

When  results  of  the  novelty  and  importance  of  those 


10 


t46  the  theory  OF  MUTATION 

which  have  been  published  by  de  Vries  are  brought 
to  our  notice,  we  are  naturally  disposed  to  reserve  our 
acceptance  of  the  conclusions  which  they  seem  to  indi- 
cate until  observations  have  been  made  in  confirma- 
tion of  them  by  some  competent  observer.  This  has 
now  been  done  by  Professor  MacDougal  at  the  New 
York  Botanic  Garden.*  MacDougal  has  carried  out 
observations  similar  to  those  above  described  upon 
the  offspring  of  seeds  sent  by  de  Vries  from  Holland, 
and  with  closely  similar  results.  Thus  he  has  observed 
all  the  new  forms  which  de  Vries  described,  as  well  as 
some  additional  ones  ; and  he  has  obtained  an  even 
higher  percentage  of  ‘ mutants  ’ than  de  Vries  him- 
self— namely,  about  3 per  cent,  of  the  total  number 
of  seedlings  grown.  This  last  result  is  probably  only 
due  to  the  application  of  more  thorough  methods  of 
investigation,  and  to  a smaller  mortality  of  the  weakest 
plants,  arrived  at  by  greater  care,  and  rendered  pos- 
sible by  the  warmer  summer  climate  and  by  American 
efficiency  in  method.  De  Vries  himself,  in  one  of  his 
later  generations,  when  particular  care  was  applied  to 
the  methods  of  cultivation,  obtained  nearly  3 per  cent, 
of  new  forms.  MacDougal  also  states  that  he  has 
observed  undoubted  cases  of  mutation  taking  place 
in  other  species  besides  (Enothera  Lamar ckiana. 

It  appears,  then,  that  there  can  be  no  doubt  about 
the  genuineness  of  the  phenomenon  described  by  de 
Vries.  But  it  is,  of  course,  quite  a different  thing  to 
assert  that  all  natural  species  arise  in  this  fashion,  and 
this  is  what  de  Vries’  theory,  as  distinguished  from 

* Reference  should  also  be  made  to  the  work  of  R.  R.  Gates ; 
see  his  ‘ Mutation  Factor  in  Evolution,’  1915. 


MUTATION  IN  OENOTHERA 


147 


his  facts,  amounts  to.  De  Vries  made  observations 
upon  a large  proportion  of  the  plants  of  his  district 
by  the  method  of  growing  great  numbers  of  their 
seedlings,  but  he  failed  to  find  the  same  phenomenon 
going  on  in  any  of  them.  He  therefore  supposes  that 
species  are  subject  to  comparatively  short  periods  of 
mutability  which  recur  at  relatively  long  intervals, 
and  that  all  the  species  he  examined  except  the 
(Enothera  were  in  this  intermediate  stable  period  of 
their  existence^  Direct  proof  of  this  suggestion  is 
naturally  out  of  the  question. 

It  will  be  well  to  summarize  briefly  the  conclusions 
at  which  de  Vries  has  arrived,  as  the  result  of  his 
observations  upon  (Enothera. 

The  following  are  the  points  to  which  he  attaches 
chief  importance  : 

1.  The  new  species  arise  suddenly  at  a single  step, 
without  transitional  forms. 

2.  They  are  usually  fully  constant  from  the  first 
moment  of  their  origin. 

3.  The  distinctive  characters  of  the  new  forms  agree 
in  kind  with  those  which  distinguish  from  one  another 
such  old  and  established  species  allied  to  (Enothera 
Laniarckiana  as  O.  biennis  and  0.  muricata.  Only  one 
of  the  new  forms — namely,  0.  nanella,  a dwarf  type — 
is  analogous  with  any  ordinary  kind  of  variety  of 
garden  origin. 

4.  A considerable  number  of  individuals  of  the  same 
sort  usually  make  their  appearance  at  the  same  period. 

5.  Although  the  new  types  vary  in  a normal  fashion, 
and  frequently  transgress  the  limits  dividing  them 

10 — 2 


148 


THE  THEORY  OF  MUTATION 


from  the  parental  type,  yet  their  first  appearance  has 
nothing  to  do  with  normal  or  continuous  variability. 

6.  The  mutations  take  place  indefinitely,  showing 
no  special  tendency  in  any  particular  direction. 

7.  The  tendency  to  mutate  recurs  periodically.  But, 
as  was  previously  stated,  there  is  no  direct  evidence 
of  this  last  supposition. 

In  addition  to  what  has  already  been  said  with 
reference  to  the  method  of  origin  of  garden  varieties  in 
general,  de  Vries  has  described  a number  of  special 
phenomena  regarding  the  behaviour  of  garden  varieties 
of  plants,  some  of  which  are  of  considerable  interest. 
Taken  together,  the  facts  substantiate  to  a great  extent 
the  view  that  selection  does  not  of  itself  lead  to  the 
production  of  specific  characters.  But  de  Vries  also 
introduces  certain  new  conceptions  which  require  to 
be  briefly  described  on  account  of  their  great  general 
interest  to  practical  breeders  and  gardeners.  They 
consist  in  the  idea  of  races  existing  intermediate  be- 
tween a species  and  a complete  variety  or  sub-type  of 
it.  Such  between-races  are  of  two  kinds,  of  which  it  is 
unusual  to  find  both  in  the  case  of  the  same  species ; 
moreover,  either  of  them  may  occur  even  when  the 
complete  variety  is  quite  unknown.  In  the  case  of  a 
half-race  a small  percentage  only  of  seedlings  is  found 
to  produce  plants  which  show  the  racial  character,  the 
remainder  being  of  the  original  specific  type ; and  even 
if  the  racial  type  is  selected  for  several  generations,  the 
percentage  of  plants  of  this  type  which  is  produced 
does  not  notably  increase.  A mid-race,  on  the  other 
hand,  can  readily  be  improved  by  selection,  and  when 


HALF  RACES 


149 


best  developed  as  a rule  either  shows  the  racial  char- 
acter in  about  half  of  the  seedlings  produced,  or  else 
exhibits  in  the  great  majority  of  its  members  a com- 
bination of  the  character  of  the  species  with  that  of 
the  race.  As  an  example,  we  may  take  the  case  of 
variegated  plants,  in  which  the  leaves  show  streaks 
or  patches  of  a yellow  colour  owing  to  the  want  of 
development  of  the  proper  green  tint.  An  ordinary 
variegated  plant,  then,  is  looked  upon  as  showing  a 
combination  of  the  green  type  with  the  yellow  char- 
acter of  a completely  modified  race — the  aurea  variety, 
although  the  latter  exists  as  such  only  in  a few  rare 
cases,  in  which  the  plants  bear  leaves  showing  no 
green  pigment  at  all.  On  the  other  hand,  many 
species  of  plants  produce  a small  proportion  of  varie- 
gated individuals  at  each  sowing,  as  is  often  the  case, 
for  example,  with  Indian  corn  ; and  this  circumstance, 
according  to  de  Vries,  indicates  the  existence  of  the 
corresponding  half-race. 

The  relative  development  of  the  two  coexisting 
characters  in  such  cases  is  highly  variable,  as  anyone 
may  observe  for  himself  in  variegated  grasses  and 
similar  plants. 

It  might  be  supposed  that  it  would  be  possible  to 
pass  from  the  species  to  the  half-race,  thence  to  the 
mid-race,  and  so  on  to  the  complete  race  simply  by 
selection.  De  Vries  shows  that  this  is  very  rarely,  if 
ever,  the  case.  He  regards  the  passage  from  a half- 
race to  a mid-race,  for  example,  as  a mutation,  and  his 
observations  seem  to  show  that  this  transition  is  not 
more  frequent  than  any  other  mutations. 


150 


THE  THEORY  OF  MUTATION 


As  a further  illustration  of  what  is  meant  by  a 
between-race,  mention  may  be  made  of  the  five-leaved 
race  of  purple  clover  {TrifoUum  fratense)  obtained  by 
de  Vries,  and  developed  by  a process  of  selection.  It 
would  appear  that  the  plants  occasionally  found  grow- 
ing wild,  which  bear  a single  four-lobed  leaf,  usually 
belong  only  to  a half-race.  De  Vries  was  fortunate 
enough  to  find  two  plants  upon  each  of  which  several 
of  the  leaves  showed  this  anomaly,  and  from  these,  by 
an  elaborate  process  of  selection  extending  over  several 
years,  a race  was  obtained,  the  leaves  of  which  in  the 
majority  of  cases  showed  five  lobes,  whilst  some  had 
six  or  seven.  Since,  however,  it  appeared  impossible 
to  get  rid  of  a certain  proportion  of  three-lobed  leaves, 
and  equally  so,  on  the  other  hand,  to  obtain  leaves 
with  more  than  seven  lobes,  de  Vries  concluded  that 
his  experiment  exemplified  the  development  of  a mid- 
race, and  not  that  of  a constant  race  or  true  variety. 

The  phenomena  described  by  de  Vries  under  the 
head  of  ‘ Between -Races  ’ are  difficult  to  understand 
until  the  reader  has  arrived  at  some  appreciation  of 
the  very  wide  differences  which  even  slight  changes  in 
external  conditions  may  make  in  the  growth  and 
habit  of  plants.  We  may  briefly  describe  one  very 
striking  example. 

The  little  water  ranunculus,  which  often  covers  areas 
of  stagnant  water  in  the  spring  vdth  a sheet  of  white 
blossom,  possesses  two  kinds  of  leaves,  the  appearance 
of  which  entirely  depends  upon  the  stimulus  of  external 
circumstances.  In  the  young  state  the  whole  plant  is 
submerged  beneath  the  surface  of  the  water,  and  bears 


Fig.  13. — Trifolium  Pratexse  Quixquefolium. 
{From  de  Vries.) 


[7'o  /ace  p.  150. 


BETWEEN  RACES 


151 

leaves  so  finely  divided  or  dissected  into  minute  seg- 
ments as  to  resemble  a camel’s-hair  pencil  when  re- 
moved from  the  water.  Sooner  or  later  the  growing 
terminal  bud  reaches  the  surface,  and  rises  above  it 
into  the  air.  As  soon  as  this  happens,  the  rudimentary 
leaves  just  beginning  to  swell  within  the  bud  entirely 
change  their  course  of  development.  They  grow  now 
into  flat-lobed  blades,  which  float  upon  the  surface  of 
the  water.  The  change  of  environment  from  water  to 
air  has  worked  such  an  alteration  in  form  that  no  one 
who  was  not  in  the  secret  would  suppose  that  these 
two  kinds  of  leaves  could  possibly  have  been  borne 
upon  the  same  plant. 

De  Vries  would  say  that  the  tendency  to  produce 
the  floating  kind  of  leaf  was  latent  in  the  submerged 
plant.  In  other  words,  the  appearance  of  any  given 
plant,  or  that  of  any  given  part  of  it,  depends  partly 
upon  its  hereditary  qualities  and  partly  to  the  external 
circumstances  to  which  it  is  submitted.  Many  other 
examples  of  similar  changes  could  be  alluded  to,  and 
one  recently  described  is  of  rather  special  interest  to 
students  of  genetics.  This  relates  to  a variety  of 
Primula  sinensis,  which,  if  kept  at  a temperature  of 
30°  C.,  in  a moist  greenhouse,  produces  red  flowers 
only  ; whilst  under  similar  conditions,  but  at  a tem- 
perature of  20°  C.,  bears  only  pure  white  blossoms. 
Another  variety  of  the  same  species  exists  which  only 
produces  white  blooms  when  exposed  to  any  condi- 
tions under  which  it  will  flower  at  all.  In  describing 
the  hereditary  difference  between  these  two  varieties, 
we  cannot  say  that  it  consists  in  the  former  having  red 


152 


THE  THEORY  OF  MUTATION 


flowers  and  the  latter  having  white.  The  difference  is 
that  the  former  variety  has  the  power  of  producing  red 
flowers  under  certain  circumstances  which  can  be  more 
or  less  rigidly  defined,  whilst  the  contrasted  type  has 
no  such  power.  Thus,  a few  degrees  difference  in 
temperature  may  determine  a marked  change  in  the 
colour  of  a flower. 

It  is  not  absolutely  certain  what  it  is  that  deter- 
mines the  difference  between  the  successive  leaves  on 
a plant  of  the  five-leaved  clover,  but  a strong  hint  is 
afforded  by  the  fact  that  the  leaves  with  the  maximum 
number  of  lobes  only  appear  when  the  plant  is  at  the 
height  of  its  vegetative  activity.  Early  in  the  season, 
and  again  towards  the  close  of  the  growing  period, 
leaves  with  fewer  lobes  are  produced.  It  would  there- 
fore seem  as  if  the  change  in  the  number  of  lobes  were 
intimately  connected  with  changes  in  the  vigour  and 
rate  of  growth  of  the  plant. 

In  the  present  chapter  we  are  dealing  particularly 
with  the  variations  of  plants,  and,  indeed,  de  Vries 
himself  has  never  applied  his  views  to  the  case  of  the 
animal  kingdom.  Although  animals  show  some  direct 
response  in  structure  and  functions  to  changes  in  their 
surroundings,  these  are  not  usually  nearly  so  extensive 
or  definite  as  the  changes  which  we  have  just  described. 
The  statement  has  even  been  made  that  one  of  the 
fundamental  differences  between  plants  and  the  higher 
animals  lies  in  the  much  greater  susceptibility  of  the 
former  to  environmental  changes. 

The  views  of  de  Vries  with  regard  to  the  actual 
origin  of  new  species  may  be  summed  up  as  follows  : 


THE  THEORY  OF  MUTATION 


153 


Bioadly  speaking,  species  arise  by  mutation,  by  a 
sudden  step  in  which  either  a single  character  or  a 
whole  set  of  characters  together  become  changed.  In 
the  former  case  a new  variety  in  the  strict  sense  of  the 
word  is  the  result  ; in  the  latter  a new  species  (accord- 
ing to  Jordan’s  definition)  is  produced. 

But  mutation  may  be  of  several  kinds.  In  the  first 
place,  an  entirely  new  character  or  set  of  characters 
may  make  its  appearance.  To  such  a phenomenon 
de  Vries  applies  the  term  of  progressive  mutation,  and 
it  is  by  steps  of  this  kind  that  he  believes  the  main 
divisions  of  the  vegetable  kingdom  to  have  been  built 
up.  In  the  case  of  such  mutations  the  new  character 
is  supposed  to  come  into  existence  first  in  a latent  or 
hidden  condition,  and  it  may  be  only  after  many 
generations  that  it  makes  its  appearance  visibly.  On 
this  view  the  period  of  mutation  is  preceded  by  a 
premutation  period,  during  which  the  appearance  of 
the  new  character  is  being  prepared  for. 

A second  method  of  species  formation,  entitled 
by  de  Vries  degressive  mutation,  is  indicated  when  a 
change  takes  place  in  the  partial  latency  of  a character. 
A completely  latent  character  is,  indeed,  unrecognisable 
as  such.  But  characters  may  also  be  only  partially 
latent,  and  in  these  cases  they  exhibit  themselves  from 
time  to  time  in  rare  individuals  in  the  form  of  sports 
or  abnormalities — a phenomenon  which  we  have  already 
seen  to  be  characteristic  of  half-races  ; indeed,  a half- 
race might  have  been  defined  as  a strain  in  which  the 
character  of  the  complete  race  is  usually  latent,  and 
only  rarely  appears.  An  active  character,  on  the  other 


154 


THE  THEORY  OF  MUTATION 


hand,  is  apparent  in  the  great  majority  of  the  indi- 
viduals of  a race.  If,  now,  a change  fi'om  latency  to 
activity  occurs  suddenly,  this  is  a form  of  mutation. 
The  reverse  case,  too,  may  occur — a character  pre- 
viously active  may  become  latent ; the  character  then 
appears  to  be  lost,  and  the  mutation  is  said  to  be 
retrogressive.  De  Vries  regards  the  great  variety  of 
allied  species  which  is  to  be  found  in  many  groups  as 
being  to  a large  extent  the  result  of  retrogressive  muta- 
tion. This  type  of  mutation  is  also  frequent  among 
cultivated  plants.  Thus,  the  appearance  of  a white 
variety  of  a species  previously  only  known  to  produce 
coloured  flowers  may  constitute  a good  example  of  a 
retrogressive  change.  Many  of  the  phenomena  some- 
what vaguely  described  by  de  Vries  as  cases  of  latency 
have  now  received  a more  precise  interpretation  in 
terms  of  the  interaction  of  invisible  factors  in  perfectly 
■definite  ways ; these  are  more  fully  described  in  the 
chapters  on  Mendelism.  Finally,  new  and  distinct 
types  may  arise  by  the  intercrossing  of  separate 
species,  but  this  is  not  regarded  by  de  Vries  as  being 
an  important  source  of  permanent  new  forms. 

Without  following  de  Vries  into  all  the  niceties  of 
his  theory  as  to  the  particular  kinds  and  methods  of 
mutations,  we  must  admit  that  his  experiments  go 
far  to  establish  the  doctrine,  in  support  of  which  a 
considerable  amount  of  evidence  had  previously  been 
accumulated,  especially  by  Bateson,  that  the  origin  of 
species  in  Nature  is  generally  a definite  process,  and 
takes  place  by  steps  of  considerable  amplitude.  What, 
then,  is  the  meaning  of  individual  differences,  of  that 


THE  THEORY  OF  MUTATION 


155 


continuous  variability  which  is  often  so  considerable, 
and  of  the  inheritance  of  this  kind  of  differences  which 
the  biometricians  have  been  at  so  much  pains  to  prove  ? 
De  Vries  points  out  that  for  no  two  plants  are  the  con- 
ditions of  life  exactly  the  same  ; a considerable  degree 
of  diversity  among  the  plants  themselves  is  therefore 
advantageous,  even  when  these  belong  to  the  same 
specific  type.  Upon  continuous  variability  depend 
local  races,  forms  adapted  to  wetter  and  drier  situa- 
tions, highland  and  lowland  races,  and  the  like,  but 
none  of  these  are  permanent.  As  regards  the  cause 
of  this  variability,  apart  from  the  effect  of  sexual 
reproduction,  which  combines  the  tendency  to  vary 
of  two  separate  parents,  de  Vries  believes  that  indi- 
vidual variability  depends  entirely  upon  nutrition  ; 
but  under  this  head  he  includes  practically  the  whole 
etivironment  of  plants — light,  space,  soil,  moisture, 
and  the  like.  Characters  acquired  in  a similar  way  by 
previous  generations  are  inherited,  and  the  effect  of 
conditions  upon  the  developing  seed  whilst  still  borne 
upon  the  parent  plant  may  be  considerable.  Thus 
easily  does  de  Vries  dispose  of  the  puzzling  question 
of  the  inheritance  or  non-inheritance  of  acquired 
characters.  Acquired  characters  are  inherited ; they 
are  not  of  any  importance  in  the  origin  of  species. 

With  regard  to  the  causes  of  mutations,  little  is 
known.  Still,  it  is  no  longer  incumbent  upon  us,  as  it 
was  a few  years  ago,  to  admit  that  we  know  nothing  at 
all  about  the  means  by  which  this  form  of  variation 
can  be  produced.  W.  L.  Tower,  in  his  ‘ Evolution  in 
Chrysomelid  Beetles,’  has  shown  that  variations  due 


156  THE  THEORY  OF  MUTATION 

to  the  influence  of  the  environment  upon  the  larval 
stages  of  the  beetle  are  not  inherited  ; but  if  the  female 
is  subjected  to  abnormal  conditions  for  a few  days  at 
the  time  when  the  eggs  are  maturing,  the  eggs  and 
larvae  being  afterwards  allowed  to  develop  in  the 
normal  environment,  a greatly  increased  number  of 
mutations  is  obtained,  the  majority  of  which  are  the 
same  as  those  found  much  more  rarely  in  Nature. 

MacDougal,  too,  has  met  with  some  success  in  the 
attempt  to  produce  mutations  artificially  in  plants. 
In  one  or  two  cases,  after  injecting  weak  solutions  of 
different  chemical  substances  into  the  young  ovaries 
of  Raimannia  and  CEnofhera,  seedlings  were  obtained 
which  differed  from  anything  previously  seen.  Up  to 
the  present  time  these  successes  seem  to  be  too  few  in 
number  to  allow  of  any  definite  conclusions  being  based 
upon  them. 

Blaringhem  has  also  recently  published  observations 
which  seem  to  show  that  in  the  maize-plant  injuries  to 
the  parent  occurring  previously  to  the  differentiation 
of  the  germ  cells  may  lead  to  permanent  modifications 
in  the  offspring.  In  neither  of  these  three  sets  of 
experiments  did  the  modification  produced  in  the  off- 
spring show  any  trace  of  an  adaptive  relation  to  the 
exciting  cause  which  operated  upon  the  parent. 

So  much  may  be  stated  in  order  to  indicate  the 
direction  in  which  research  is  proceeding.  In  the 
course  of  another  decade  we  may  hope  perhaps  to 
find  out  something  more  about  the  natural  and  artificial 
production  of  mutations. 


THE  THEORY  OF  MUTATION 


157 


According  to  the  view  upheld  by  Wallace,  Weismann, 
and  others,  the  actual  origin  of  specific  distinctions 
takes  place  by  natural  selection  acting  upon  individual 
differences  ; and  in  this  case  it  is  to  be  observed  that 
it  is  the  struggle  between  individuals  of  the  same  species 
which  is  of  primary  importance.  On  the  mutation 
theory  it  is  only  the  competition  between  allied  species 
which  interests  us  from  the  point  of  view  of  evolution. 
Natural  selection  is  thus  regarded  as  having  no  influ- 
ence in  the  formation  of  species  themselves.  On  the 
other  hand,  the  gaps  existing  between  genera  and  still 
larger  groups,  such  as  families  and  classes,  are  still  sup- 
posed to  be  due  to  the  destructive  action  of  natural 
selection  determining  the  survival  of  the  fittest  species, 
so  that  this  principle  is  by  no  means  ousted  from  its 
prominent  position  in  the  philosophy  of  evolution  even 
when  the  latter  is  expounded  by  the  mutationist. 

One  further  point.  On  the  theory  of  mutation  the 
survival  of  useless  structures  becomes  readily  com- 
prehensible. Indeed,  a structure  which  is  actually  of 
the  nature  of  a handicap  to  its  possessor  may  fail  to 
cause  extinction  if  it  is  either  combined  with  a vigorous 
constitution,  or  correlated  with  other  characteristics 
which  are  sufficiently  useful  to  make  up  for  the  dis- 
advantages entailed.  The  survival  of  many  apparently 
useless  and  some  apparently  harmful  structures  is  very 
difficult  to  understand  on  the  hypothesis  of  a con- 
tinuous evolution  by  the  survival  of  the  fittest  indi- 
viduals. This  is  an  argument  upon  which  de  Vries 
lays  considerable  stress,  although  it  may  be  pointed 


158 


THE  THEORY  OF  MUTATION 


out  that  it  is  usually  very  difficult  to  form  a judgment 
as  to  the  real  usefulness  or  otherwise  of  organs. 

[Note. — A most  remarkable  series  of  mutations  has 
been  obtained  by  Prof.  T.  H.  Morgan,  of  New  York,  in 
the  little  Fruit  Fly  Drosophila.  A description  of  many 
of  them,  and  of  the  manner  of  their  inheritance,  will  be 
found  in  his  books  ^ A Critique  of  the  Theory  of  Evolu- 
tion,' Princeton  University  Press,  1916,  and  ' The 
Physical  Basis  of  Heredity,'  1919.] 

BIBLIOGRAPHY. 

Bateson,  W.  : Materials  for  the  Study  of  Variation,  1894. 
Vries,  H.  de  : Die  Mutationstheorie,  1901. 

,,  Species  and  Varieties,  their  Origin  by  Mutation, 

1905. 

MacDougal,  D.  T.  : Various  papers  published  by  Carnegie 
Institution. 

Tower,  W.  L.  : An  Investigation  of  Evolution  in  Chrysomelid 
Beetles  of  the  Genus  Leptinotarsa,  1906. 

Gates,  R.  R.  : The  Mutation  Factor  in  Evolution,  1915* 


THE  LIBRARY 
Of  THE 

UNIVERSITY  OF  ILLINOIS 


Joseph  Gottlieb  Kolreuter,  1733-1806. 
{Ajtev  an  engraving  by  J.  Cederquist.  ) 


\To  face  p.  159. 


CHAPTER  VI 


THE  OLDER  HYBRIDISTS 

There  is  one  side  of  the  practical  study  of  heredity 
which  dates  back  to  the  middle  of  the  seventeenth 
century — namely,  that  branch  of  the  subject  which  is 
concerned  with  the  hybridizing  or  artificial  cross- 
breeding of  different  species  and  varieties  of  plants. 
Quite  recently  the  great  importance  which  attaches 
to  this  method  of  study  has  been  realized  once  more, 
and  the  interest  thus  awakened  has  led  to  a closer 
examination  of  the  accounts  of  experiments  under- 
taken a century  or  more  ago,  with  the  result  of  showing 
that  much  of  the  work  then  carried  out  in  this  direc- 
tion had  attained  to  quite  an  astonishing  degree  of 
excellence.  In  the  brief  sketch  of  the  history  of 
hybridizing  work  here  following,  account  will  be  taken 
almost  exclusively  of  experiments  of  which  the  in- 
terest is  not  historical  only,  but  which  possess  an 
actual  scientific  value.  Amongst  other  matters  of 
interest,  it  will  be  found  that  more  than  one  observer 
came  very  near  to  anticipating  Mendel’s  epoch- 
making  discovery,  and  thus  arriving  at  the  clue  which 
should  unravel  almost  all  the  complex  problems  which 
beset  the  early  hybridizers. 

159 


i6o 


THE  OLDER  HYBRIDISTS 


Following  the  modern  usage,  we  shall  apply  the 
term  ‘ hybrid  ’ to  all  individuals  arising  from  a cross 
between  parents  which  belong  to  distinct  groups,  no 
matter  whether  these  groups  are  separated  as  distinct 
genera  or  species,  or  whether  they  are  regarded  as 
representing  only  different  races  or  varieties.  This 
wide  interpretation  of  the  term  hybrid  has  only  re- 
cently been  reintroduced.  The  use  to  which  it  has 
returned  is,  indeed,  the  original  one  ; but  many  inter- 
mediate writers,  including  Darwin,  confined  the  em- 
ployment of  this  expression  to  cases  of  crossing  between 
species,  and  applied  the  word  ‘ mongrel  ’ to  the  off- 
spring of  crosses  between  races  or  varieties  of  the  same 
species.  Darwin,  however,  did  not  regard  species  as 
differing  in  kind  from  varieties,  and  he  even  particu- 
larly emphasized  the  smallness  of  the  distinction  which 
can  be  drawn  between  the  behaviour  and  properties 
of  hybrids  and  mongrels  respectively.  Indeed,  he 
came  to  the  highly  important  conclusion  that  the  laws 
of  resemblance  between  parents  and  their  children  are 
the  same,  whatever  may  be  the  amount  of  difference 
between  the  parents  in  question — whether,  that  is  to 
say,  they  are  distinguished  only  by  individual  differ- 
ences, or  whether  they  belong  to  separate  varieties  or 
even  species.  We  have  already  seen  that  the  more 
recent  facts  of  biometry  point  strongly  towards  the 
conclusion  that  individual  and  race  differences  are 
inherited  at  approximately  the  same  rate.  It  seems, 
however,  to  be  at  present  somewhat  doubtful  whether 
all  sorts  of  specific  differences  follow  the  sar  e law  of 
propagation  on  cross-breeding. 


KOLREUTER 


i6i 

Between  1760  and  1766  Joseph  Gottlieb  Kolreuter 
carried  out  the  first  series  of  systematic  experiments 
in  plant  hybridization  which  had  ever  been  under- 
taken.' These  experiments  not  only  established  with 
certainty  for  the  first  time  the  fact  that  the  seeds  of 
plants  are  produced  by  a sexual  process  comparable 
with  that  known  to  occur  in  animals,  but  also  led  to  a 
knowledge  of  the  general  behaviour  of  hybrid  plants, 
which  was  scarcely  bettered  until  Mendel  made  his 
observations  a century  afterwards. 

Kolreuter  found  that  the  hybrid  offspring  of  tw'o 
different  plants  generally  took  as  closely  after  the 
plant  which  yielded  the  pollen  as  after  that  upon 
which  the  actual  hybrid  seed  was  borne.  Indeed,  he 
found  that  it  made  little  or  no  difference  to  the  ap- 
pearance of  the  hybrid  which  of  the  parental  species 
was  the  pollen-parent  (male),  and  which  the  seed- 
parent  (female) — that  is  to  say,  in  the  case  of  plants 
the  result  of  reciprocal  crosses  is  usually  identical. 
Thus,  for  the  first  time  it  was  definitely  shown  that 
the  pollen-grain  plays  just  as  important  a part  in 
determining  the  characters  of  the  offspring  as  does  the 
ovule  which  the  pollen-grain  fertilizes.  This  was  a 
wholly  novel  idea  in  Kolreuter’s  time,  and  the  fact  was 
scarcely  credited  by  his  contemporaries. 

Kolreuter  had  no  means  of  discovering  that  the 
contents  of  a single  pollen-grain  unite  with  the  con- 
tents of  a single  ovule  in  fertilization.  But  he  ascer- 
tained by  experiments  that  more  than  thirty  seeds 
might  be  made  to  ripen  by  the  application  of  between 

fifty  and  sixty  pollen-grains  to  the  stigma  of  a par- 

II 


i62 


THE  OLDER  HYBRIDISTS 


ticular  flower,  so  that,  if  he  had  had  any  hint  of  the 
actual  microscopic  processes  of  fertilization,  he  w'ould 
have  been  quite  prepared  for  the  more  fundamental 
discovery. 

Kolreuter,  indeed,  believed  that  the  act  of  fertiliza- 
tion consisted  in  the  intimate  mingling  together  of  two 
fluids,  the  one  contained  in  the  pollen-grain,  and  the 
other  secreted  by  the  stigma  of  the  plant.  The  mingled 
fluids,  he  supposed,  next  passed  down  the  style  into 
the  ovary  of  the  plant,  and  arriving  at  the  unripe 
ovules,  initiated  in  them  those  processes  which  led  to 
the  formation  of  seeds.  In  this  belief  Kolreuter  simply 
followed  the  animal  physiologists  of  his  time,  who 
looked  upon  the  process  of  fertilization  in  animals  as 
taking  place  by  a similar  mingling  together  of  two 
fluids.  Now  that  we  know  that  fertilization  consists 
essentially  in  the  intimate  union  of  the  nuclei  of  two 
cells,  one  of  which,  in  the  case  of  plants,  is  the  ovum 
contained  within  the  ovule,  whilst  the  other  is  repre- 
sented by  one  of  a few  cells  into  which  the  contents 
of  the  pollen-grain  divide,  we  can  understand  more 
clearly  the  bearing  of  Kolreuter’s  observation.  And 
it  is  greatly  to  this  eminent  naturalist’s  credit  that  he 
succeeded  in  carrying  out  his  observations  with  so 
much  accuracy,  when  the  full  meaning  of  those 
observations  was  of  necessity  hidden  from  his  com- 
prehension. 

Kolreuter  was  the  first  to  observe  accurately  the 
different  ways  in  which  pollen  can  be  naturally  con- 
veyed to  the  stigma  of  a flower.  This  may  take  place 
either  by  the  pollen-grains  falling  directly  upon  the 


KOLREUTER 


163 


stigma,  or  by  the  agency  of  the  wind,  or,  lastly,  the 
pollen  may  be  carried  by  insects  visiting  the  flowers. 
And  he  recognised  many  features  characteristic  of 
flowers  apt  to  be  fertilized  in  one  or  other  of  these 
ways  in  particular.  Thus  he  was  aware,  for  example, 
of  the  nature  and  use  of  the  nectar  which  so  many 
flowers  produce — namely,  that  it  is  the  substance  from 
which  the  bees — by  far  the  most  diligent  visitors  of 
flowers— obtain  their  honey. 

Curiously  enough,  Kolreuter  was  not  aware  of  the 
existence  of  any  natural  wild  hybrid  plants.  But  he 
was  quite  right  in  contending  that  supposed  examples 
of  such  hybrids  required  for  their  substantiation  the 
experimental  proof,  which  could  only  be  afforded  by 
making  actual  artificial  crosses  between  the  putative 
parent  species. 

The  first  hybrid  made  artificially  by  Kolreuter  was 
obtained  in  1760  by  applying  the  pollen  of  Nicotiana 
paniculata  to  the  stigma  of  Nicotiana  rustica.  The 
hybrid  offspring  of  this  cross  showed  a character  inter- 
mediate between  those  of  the  two  parent  species  in 
almost  every  measurable  or  recognisable  feature,  with 
a single  notable  exception.  This  exception  was 
afforded  by  the  condition  of  the  stamens  and  of  the 
pollen  grains  produced  by  the  hybrids.  These  organs 
were  so  badly  developed  that  in  all  the  earlier  experi- 
ments self-fertilization  of  the  hybrid  plants  yielded  no 
good  seed  at  all,  nor  were  the  pollen  grains  of  the 
hybrid  any  more  effective  when  applied  to  the  stigmas 
of  either  of  the  parent  species.  On  the  other  hand, 
when  pollen  from  either  parent  was  a.pplied  to  the 

II — 2 


164 


THE  OLDER  HYBRIDISTS 


stigmas  of  the  hybrid  plants,  a certain  number  of  seeds 
capable  of  germination  was  obtained,  although  this 
number  was  much  smaller  than  in  the  case  of  normal 
fertilization  of  either  parent  species.  This  partial 
sterility,  affecting  in  particular  the  stamens  and  the 
pollen  which  they  produce,  is  a feature  common  to  the 
majority  of  hj^brids  between  different  natural  species. 
Many  such  hybrids,  indeed,  are  altogether  sterile,  so 
that  a further  generation  cannot  in  any  way  be  ob- 
tained from  them.  On  the  other  hand,  the  members 
of  different  strains  or  varieties  which  have  arisen  under 
cultivation  yield,  as  a rule,  when  crossed  together  off- 
spring which  are  perfectly  fertile. 

In  subsequent  years  Kolreuter  w'as  able  to  obtain  a 
very  few  self-fertilized  offspring  from  hybrids  of  the 
same  origin  as  the  above.  The  resulting  plants  were 
described  as  resembling  their  hybrid  parent  so  closely 
as  to  be  practically  indistinguishable  from  it. 

The  offspring  obtained  by  crossing  the  hybrid  plants 
with  pollen  from  either  parent  showed  in  each  case  a 
form  more  or  less  intermediate  between  that  of  the 
original  hybrid  and  that  of  the  parent  species  from 
which  the  pollen  was  derived.  But  the  plants  were 
not  all  alike  in  this  respect,  some  of  them  being  much 
more  like  the  parent  species  than  others,  and  some, 
again,  varying  in  other  directions.  There  were  also 
considerable  differences  between  the  different  indi- 
viduals in  respect  of  fertility,  so  that  some  of  the 
plants  were  more  and  some  less  sterile  than  the  original 
hybrids.  Also,  there  was  some  tendency  to  the  produc- 
tion of  malformations  of  the  flowers  and  other  parts. 


KOLREUTER 


165 

One  of  the  most  noted  of  Kolreuter’s  experiments 
was  that  which  consisted  in  repeatedly  recrossing  a 
hybrid  plant  with  one  of  the  parent  species  from  which 
the  hybrid  was  derived.  By  continuing  to  pollinate 
the  members  of  one  generation  after  another  with  the 
pollen  of  the  same  parent  species,  plants  were  at  last 
arrived  at  which  were  indistinguishable  from  the  parent 
in  question.  We  shall  return  to  this  fact  later  on, 
when  the  reader  will  be  in  a position  to  appreciate  its 
importance  more  fully. 

Kolreuter  found  that  the  result  of  reciprocal  crosses 
is  usually  identical — that  is  to  say,  the  offspring  ob- 
tained by  fertilizing  a plant  A with  pollen  from  a 
plant  B are  not  to  be  distinguished  from  those  ob- 
tained when  B is  fertilized  with  the  pollen  of  A.  But 
the  two  opposite  processes  of  fertilization  are  not 
always  equally  easy  to  carry  out.  An  extreme  instance 
of  this  circumstance  was  met  with  in  the  case  of  the 
genus  Mirabilis.  Mirabilis  jalapa  was  easily  fertilized 
with  pollen  from  M.  longiflora.  During  eight  years 
Kolreuter  made  rhore  than  two  hundred  attempts  to 
effect  the  reverse  cross,  but  without  success. 

It  was  shown  by  Kolreuter  that  hybrids  between 
different  races  or  varieties  of  the  same  species  are 
usually  much  more  fertile  than  hj^brids  obtained  by 
crossing  distinct  species.  Indeed,  he  believed  that 
varieties  of  a single  species  were  in  all  cases  perfectly 
fertile  together,  whilst  hybrids  between  species  always 
showed  some  degree  of  sterility.  But  in  this  case  Kol- 
reuter based  his  definition  of  a species  upon  the  very 
point  at  issue,  and  when  he  found  forms,  which  other 


i66 


THE  OLDER  HYBRIDISTS 


botanists  regarded  as  good  species,  to  be  perfectly 
fertile  together,  he  immediately  regarded  them  as 
being  only  varieties  of  a single  species. 

One  curious  point  is  worth  quoting  in  this  connec- 
tion. Five  varieties  of  Nicotiana  tabacum  were  found 
to  be  perfectly  fertile  with  one  another,  but  when 
crossed  with  Nicotiana  glutinosa  one  of  them  was 
found  to  be  distinctly  less  sterile  than  the  rest. 

Another  interesting  point  observed  by  Kolreuter 
was  the  fact  that  hybrid  plants  often  exceed  their 
parents  in  luxuriance  of  growth.  Upon  this  fact,  as 
we  shall  see  later  on,  Knight  and  afterwards  Darwin 
based  theoretical  conclusions  of  considerable  impor- 
tance in  connection  whh  the  problem  of  sex. 

Thomas  Andrew  Knight,  who  was  also  a botanist  of 
high  reputation  in  other  fields,  was  the  earliest  observer 
to  lay  stress  upon  the  practical  aspect  of  the  study  of 
hybrids,  and  he  occupied  himself  to  a considerable 
extent  with  the  improvement  of  useful  races  of  plants 
by  cross-breeding.  Breeders  of  animals  had  already 
made  important  improvements  by  the  method  of  inter- 
crossing different  races,  and  selecting  the  most  notable 
types  which  made  their  appearance  in  consequence, 
when  Knight  bethought  him  of  applying  the  same 
principles  to  the  improvement  of  plants,  and  particu- 
larly of  fruit-trees. 

Knight  also  carried  out  a series  of  experiments  with 
domestic  peas,  the  results  of  which  were  published  in 
1779.  These  experiments  have  a particular  interest 
from  the  historical  point  of  view,  since  it  was  by  dint 
of  similar  experiments  upon  the  same  kind  of  plants 


KNIGHT 


167 


that  Mendel’s  law  was  afterwards  discovered.  This 
very  discovery  might  even  have  befti  made  by  Knight 
himself,  if  he  had  only  realized  the  importance  of  ascer- 
taining on  a large  scale  the  numerical  proportions  in 
which  the  different  kinds  of  plants,  arising  in  the 
second  generation  from  the  crosses,  made  their  appear- 
ance. Unfortunately,  this  particular  form  of  inquiry 
never  seems  to  have  occurred  to  him. 

Knight’s  experiments  were  made  with  a different 
object  in  view — namely,  that  of  discovering  whether 
a cross  with  a distinct  race  would  provide  the  stimulus 
necessary  to  restore  its  lost  vigour  to  a strain  of  plants 
which  was  supposed  to  have  become  debilitated,  owing 
to  its  members  having  been  bred  exclusively  by  self- 
pollination  for  a long  series  of  generations. 

The  result  of  the  experiments  undoubtedly  estab- 
lished the  fact  that  in  some  cases  the  hybrid  offspring 
of  two  distinct  races  shows  a more  vigorous  habit  of 
growth  than  either  of  the  parental  types.  The  follow- 
ing extract  from  Knight’s  own  account  will  indicate 
the  nature  of  the  experiments  upon  which  his  con- 
clusions rest : 

‘ By  introducing  the  farina  of  the  largest  and  most 
luxuriant  kinds  into  the  blossoms  of  the  most  diminu- 
tive, and  by  reversing  this  process,  I found  that  the 
powers  of  the  male  and  female,  in  their  effects  on  the 
offspring,  are  exactly  equal.  The  vigour  of  the  growth, 
the  size  of  the  seeds  produced,  and  the  season  of 
maturity,  were  the  same  though  the  one  was  a very 
early,  the  other  a very  late  variety.  I had  in  this 
experiment  a striking  instance  of  the  stimulative  effects 


i68 


THE  OLDER  HYBRIDISTS 


of  crossing  the  breeds,  for  the  smallest  variety,  whose 
height  rarely  exceeded  two  feet,  was  increased  to  six 
feet,  whilst  the  height  of  the  large  and  luxuriant  kind 
was  very  little  diminished.’ 

We  shall  see,  however,  that  the  phenomenon  last 
alluded  to  admits  of  a different  interpretation. 

It  was  upon  the  somewhat  slender  basis  afforded 
by  this  experiment  that  the  generalization  known  as 
the  Knight -Darwin  law  was  originally  established. 
Knight’s  own  expression  of  this  idea  was  to  the  effect 
that  ‘ Nature  intended  that  a sexual  intercourse  should 
take  place  between  neighbouring  plants  of  the  same 
species.’  And  the  same  conclusion  was  expressed  still 
more  forcibly  by  Darwin  in  the  aphorism  : ‘ Nature 
abhors  perpetual  self-fertilization.’  But  although  it 
may  be  true  that  in  a considerable  number  of  cases 
advantages  are  gained  from  the  process  of  cross- 
fertilization between  different  members  of  the  same 
species,  which  do  not  accrue  when  self-fertilization 
takes  place,  yet  several  cases  are  now  known  in  which 
self-fertilization  really  does  seem  to  be  indefinitely 
continued, 

Knight  crossed  a pea  having  white  flowers  and  seed- 
coats,  and  green  stems,  with  one  in  which  the  flowers 
and  stems  were  coloured  purple,  and  the  seeds  grey. 
The  seeds  immediately  resulting  from  the  cross  were 
unchanged  in  appearance,  but  the  plants  arising  from 
these  took  closely  after  their  coloured  male  parent. 
On  crossing  the  cross-bred  plants  once  more  with  a 
white  strain  a certain  proportion  of  white  plants  was 
again  obtained,  though  what  that  proportion  was 


KNIGHT 


169 


Knight  failed  to  notice.  He  observed,  however,  that 
white  crossed  by  a purple  strain  invariably  gave  purple, 
whilst  the  cross-bred  purples,  when  crossed  again  with 
white,  yielded  some  white  and  some  purple  plants. 

In  1822  John  Goss  recorded  the  fact  that  a ‘ blue  ’ 
pea  crossed  with  a ‘ white  ’ yielded  from  the  crossed 
flowers  pods  with  white  seeds  only,  the  seeds  contained 
in  other  pods  upon  the  same  plant  being,  of  course, 
blue.  The  plants  produced  from  the  white  seeds  bore 
some  pods  with  all  blue,  some  with  all  white,  and  many 
pods  with  both  white  seeds  and  blue  ones  ; and  a 
coloured  plate  is  given  which  shows  one  of  the  latter 
pods  together  with  its  contents.  The  blue  seeds,  when 
sown  separately,  yielded  plants  which  produced  blue 
seeds  only,  but  plants  arising  from  the  white  seeds 
yielded  a mixture  of  blue  and  white  seeds. 

Knight  pointed  out  quite  correctly  that  the  colours 
of  the  seeds  which  are  here  referred  to  are  occasioned 
by  the  colour  of  the  cotyledons  or  seed-leaves  of  the 
pea,  which  are  visible  through  the  semitransparent 
seed-coat.  Green  cotyledons  give  rise  in  this  way  to 
a bluish  appearance,  whilst,  when  the  cotyledons  are 
yellow,  the  resulting  appearance  of  the  seed  is  described 
as  whitish. 

The  Hon.  and  Rev.  W.  Herbert  was  another  observer 
who  made  many  important  experiments  in  hybridiza- 
tion towards  the  beginning  of  the  nineteenth  century. 
These  led  him  to  the  conclusion  that  Kolreuter  and 
Knight  were  wrong  in  their  assertion  that  hybrids 
between  distinct  species  were  always  sterile.  Herbert 
considered  that  only  generic  or  family  types  were 


170 


THE  OLDER  HYBRIDISTS 


constantly  sterile,  and  this  led  him  to  the  further 
conclusion,  now  believed  to  be  erroneous,  that  the 
separate  genera  or  families  were  those  which  were 
originally  created,  whilst  he  believed  that  the  separate 
species  of  the  same  genus  arose  from  a single  original 
type  by  a genuine  process  of  evolution. 

The  most  prolific  in  work  of  all  the  hybridists,  how- 
ever, was  undoubtedly  Carl  Friedrich  v.  Gaertner 
(1772  - 1850).  Gaertner  made  a great  number  of 
crosses  between  species  belonging  to  all  sections  of 
the  natural  system,  and  his  book,  published  in  1849, 
contains  a great  mass  of  valuable  information.  Gaert- 
ner’s  theoretical  conclusions,  for  the  most  part,  only 
amplify  and  confirm  those  of  Kolreuter,  upon  whom 
in  this  direction  he  made  but  little  advance. 

C.  Naudin’s  essay,  entitled  ‘ New  Researches  on 
Hybridity  in  Plants,’  made  its  appearance  in  1862. 
The  author  pointed  out  that  the  facts  of  the  return 
of  hybrids  to  the  specific  forms  of  their  parents,  when 
repeatedly  crossed  with  the  latter,  are  naturally  ex- 
plained by  the  hypothesis  of  the  disjunction  of  the 
two  specific  essences  in  the  pollen  grains  and  ovules 
of  the  hybrid.  The  idea  may,  perhaps,  be  made  some- 
what clearer  as  follows  : Let  us  consider  the  case  of  a 
species  A crossed  with  another  species  B.  Naudin 
supposes  that  some  of  the  pollen  grains  and  ovules  of 
the  hybrid  plant  will  be  potentially*  of  the  exact 

* When  it  is  said  that  a pollen  grain  or  ovule  potentially 
resembles  the  species  A,  it  is  meant  that  the  germ-cell  in 
question  is  of  such  a kind  that,  when  united  with  one  derived 
from  an  ovule  or  pollen  grain  of  similar  constitution,  it  would 
give  rise  to  a plant  exactly  resembling  A. 


NAUDIN 


171 

character  of  one  species  (A),  whilst  others  will  bear 
no  potential  resemblance  to  A,  but  will  be  precisely 
similar  in  nature  to  the  ovules  and  pollen  grains  of 
the  pure  species  B.  In  cases  where  this  separation  of 
the  materials  representing  the  two  types  in  a potential 
condition  is  complete,  forms  exactly  resembling  the 
parents  might  be  obtained.  As  we  shall  see,  this 
hypothesis  makes  a remarkably  near  approach  to  that 
of  Mendel ; and  the  importance  of  the  fact  that  the 
first  hybrid  generation  is  generally  uniform,  as  con- 
trasted with  the  diversity  of  types  often  appearing 
in  the  second  generation,  is  clearly  recognised  by 
Naudin.  This  observer  considered  the  hybrid  in  the 
adult  state  to  consist  of  an  aggregate  of  particles, 
homogeneous  and  characteristic  of  a single  species 
when  taken  separately,  but  mingled  in  various  pro- 
portions in  the  organs  of  the  hybrid,  which  is  thus 
looked  upon  as  a kind  of  living  mosaic. 

Great  numbers  of  observations  upon  the  character- 
istics and  behaviour  of  hybrid  plants  and  animals  have 
been  from  time  to  time  recorded,  and  the  preceding 
pages  contain  only  a brief  selection  of  such  facts  as 
are  most  necessary  for  a proper  understanding  of 
modem  work  in  hybridization.  Until  quite  recently 
the  laws  of  transmission  of  characters  in  hybrids  were 
still  completely  hidden.  The  facts  were  wonderful 
enough,  but  they  showed  no  signs  of  falling  into  an 
orderly  arrangement.  In  the  next  chapter  it  will  be 
our  business  to  describe  the  remarkable  discovery 
which  has  introduced  order  into  this  previously  chaotic 
region,  and  which  has  enabled  a few  workers  to  estab- 


172 


THE  OLDER  HYBRIDISTS 


lish  in  half  a dozen  years  the  foundations  of  a great 
science,  the  importance  of  which  is  now  becoming 
generally  realized. 

[Note. — In  the  earlier  editions  mention  was  made 
of  Millardet’s  ‘ false  hybrids  ’ between  species  of 
strawberries  (1894),  in  which  it  appeared  that  which- 
ever way  the  cross  was  made,  the  offspring  resembled 
the  maternal  species  and  were  said  to  breed  true. 
Subsequent  work  by  other  investigators  has  not  com- 
pletely confirmed  Millardet’s  observations,  and  as  the 
facts  are  still  doubtful,  it  has  been  thought  better  to 
omit  the  passage  from  the  text.  For  somewhat 
analogous  phenomena  in  Tripsacum  and  Euchlcsna 
(genera  allied  to  maize),  see  Collins  and  Kempton, 
Journal  of  Heredity,  1916,  p.  106.] 


THE  OLDER  HYBRIDISTS 


X73 


BIBLIOCxRAPHY. 

Kolreuter,  J.  G.  : Vorlaufige  Nachricht  von  einigen  das 
Geschlecht  der  Pfianzen  betreffenden  Versuchen  und 
Beobachtungen,  1761. 

Gaertner,  C.  F.  von  : Bastarderzengung,  1849. 

Focke,  W.  O.  : Die  Pflanzen-Mischlinge,  1881. 

Wichura,  M.  : Die  Bastardbefruchtiing  im  Pflanzenreiche, 
1865. 

Darwin,  C.  ; The  Effects  of  Cross  and  Self  Fertilization. 


CHAPTER  VII 


MENDELISM 

We  have  already  had  occasion  to  point  out  how  im 
port  ant  it  is,  when  engaged  upon  questions  of  heredity, 
not  to  treat  whole  animals  or  plants  as  units,  but  to 
deal  with  their  separate  characters  one  at  a time.  In 
the  course  of  the  present  chapter  the  reason  for  pro- 
ceeding in  this  way  will  appear  more  clearly,  and  we 
shall  find  that  the  adoption  of  this  method  is  fully 
justified  by  the  results  which  it  enables  us  to  obtain, 
and  which  could  not  have  been  arrived  at  in  any  other 
way.  We  shall  also  find  reasons  for  believing  that 
this  method  is  the  correct  one  from  a theoretical  point 
of  view. 

Naturally,  considerable  care  is  necessary  in  deter- 
mining what  are  and  what  are  not  separable  characters. 
At  the  outset  it  is  not  always  possible  to  make  this 
discrimination  with  certainty,  but  during  the  course 
of  the  experiments  which  follow  it  is  almost  always 
possible  to  arrive  at  a clear  definition  of  each  character, 
and  in  many  cases  the  distinction  of  characters  is  quite 
obvious  from  the  beginning. 

Up  to  the  present  time  the  experimental  study  of 
heredity  by  the  methods  of  definite  breeding  has  yielded 

174 


Gregor  Johann  Mendel. 


[To /ace  p.  174. 


the 

Of  THE 

UHWERSaT  Of  lUlHOlS 


MENDELISM 


175 


clear  and  definite  information  only  when  applied  to 
cases  where  clearly  definable  characters  have  distin- 
guished the  parental  forms  examined.  This,  however, 
is  in  great  part  due  to  the  fact  that  the  experimental 
method  has  hitherto  been  little  used  in  dealing  with 
characters  of  a less  definite  nature.  The  science  is 
still  in  its  infancy,  and  attention  has  naturally  been 
first  paid  to  the  simpler  problems  which  it  affords. 
The  difficulties  of  treatment  which  confront  those  who 
would  deal  with  highly  variable  characters  and  those 
of  a ‘ more  or  less  ’ nature  are  considerable,  although 
there  is  no  reason  for  supposing  that  such  problems  are 
insuperable.  As  we  have  seen,  however,  the  majority 
of  characters  which  distinguish  species  or  races  from 
one  another  appear  to  be  of  a perfectly  definite  descrip- 
tion, so  that  the  limitation  just  referred  to  is  not  so 
serious  as  might  appear  at  first  sight.  The  recent 
cevival  of  work  upon  the  subject  of  inheritance  by 
the  use  of  breeding  methods  has,  as  a matter  of  fact, 
already  been  rewarded  with  results  as  valuable  and 
as  clear  as  could  possibly  have  been  anticipated — 
results  which  are  sufficient  in  themselves  to  show  that 
the  discovery  made  by  Mendel  was  of  an  importance 
little  inferior  to  those  of  a Newton  or  a Dalton. 

It  is  important  to  remember  that  every  animal  or 
plant,  which  has  come  into  existence  in  the  ordinary 
way  through  sexual  generation,  owes  its  individuality 
to  the  mingled  natures  of  two  separate  parents.  The 
following  lines,  in  which  the  poet  Goethe  speaks  of 
his  own  hereditary  endowment,  have  been  quoted 
more  than  once  in  this  connection  ; 


176 


MENDELISM 


‘ Vom  Vater  hab’  ich  die  Statur, 

Des  Lebens  emstes  Filhren, 

Vom  Mbtterchen  die  Frohnatur 
Und  Lust  zu  fabulieren.’ 

In  such  a case  we  must  always  look  upon  the  corre- 
sponding character  of  the  second  parent  as  existing 
in  the  offspring  side  by  side  with  the  character  which 
finds  expression,  only  the  former  is  overpowered  by 
the  latter,  and  forced  to  remain  invisible.  That  the 
hidden  character  is  nevertheless  actually  present  is 
shown  by  the  fact  that  a feature  characteristic  of  a 
particular  grandparent,  which  did  not  appear  in  the 
parent,  may  reappear  in  the  child.  For  instance,  a 
characteristic  masculine  feature  of  the  maternal  grand- 
father may  thus  make  its  appearance  in  the  son. 

It  is  found  that  any  individual  may  be  looked  upon 
as  being  to  a large  extent  an  aggregation  of  separate 
characteristics.  In  a pair  of  allied  races  a great 
number  of  the  separate  characters  are  the  same  in 
the  two  cases,  the  distinction  between  the  two  forms 
depending  upon  a few  definite  features  only.  The 
majority  of  salient  characteristics  are  identical  in  such 
a pair,  but  some  of  the  corresponding  characters  are 
opposed.  Thus  in  different  races  of  mankind  com- 
plexions may  be  dark  or  fair,  eyes  blue  or  brown, 
hair  straight  or  curly,  and  the  like.  Now  the  off- 
spring of  parents  which  had  smooth  and  curly  hair 
respectively  might  exhibit  smooth  or  curly  or  inter- 
mediate (wavy)  hair,  according  as  one  or  the  other 
character,  or  both  in  combination,  made  its  presence 
obvious ; for  in  the  simplest  cases  both  will  necessarily 


UNIT  CHARACTERS 


177 


be  present,  though  one  may  be  hidden.  What  will 
happen  in  the  grandchildren  ? 

The  manner  in  which  characters  comparable  with 
the  above  are  actually  transmitted  has  been  worked 
out  in  the  case  of  many  races  of  animals  and  plants, 
and  in  cases  where  experimental  matings  can  be 
readily  carried  out,  and  a large  number  of  offspring 
reared,  it  is  found  that  a simple  rule  applies  which 
holds  good  in  every  example  thoroughly  examined 
hitherto.  This  law  was  discovered  by  Mendel  about 
the  year  1865,  and  has  since  been  called  by  his  name. 
Before  enunciating  it  we  shall  consider  the  informa- 
tion afforded  by  the  case  of  a single  pair  of  simple 
characters.  Afterwards  we  shall  endeavour  to  show 
the  application  of  the  law  to  the  more  complex  cases 
in  which  combinations  of  characters  are  concerned. 

A grain  of  Indian  corn  or  maize  contains  a germ  or 
embryo,  which  under  suitable  conditions  will  give  rise 
to  the  future  plant.  The  embryo  is  surrounded  by  a 
certain  amount  of  reserve  food  material  constituting 
the  endosperm — a store  which  is  made  use  of  by  the 
young  plant  during  its  germination.  The  embryo 
arises  as  the  result  of  a process  of  fertilization  which 
takes  place  in  the  following  manner  : The  ovum,  or 
female  cell  hidden  in  a flower,  contains  a nucleus,  and 
this  on  fusion  with  one  of  the  nuclei  derived  from  a 
grain  of  pollen  initiates  the  vital  processes  which  lead 
to  the  development  of  an  embryo  plant. 

Nuclei  are  the  central  and,  from  the  point  of  view  of 

heredity,  the  most  important  parts  of  cells — ^the  con- 

12 


178 


MENDELISM 


stituent  units  of  the  plant  body.  The  cells  which, 
together  with  their  nuclei,  take  part  in  the  process  of 
fertilization  are  known  as  gametes,  or  germ-cells—malQ 
and  female  respectively,  the  latter  being  the  ovum. 

It  is  less  generally  known  that  the  endosperm  of  a 
grain  of  Indian  corn  arises  by  a very  similar  process 
to  the  one  which  gives  rise  to  the  embryo  itself.  A 
second  nucleus  derived  from  the  same  pollen  grain 
fuses  with  a nucleus  situated  near  the  ovum,  and  to 
the  product  of  this  fusion  the  endosperm  owes  its 
origin.  It  is  further  found,  so  far  at  least  as  those 
characters  are  concerned  to  which  we  shall  at  present 
confine  our  attention,  that  these  two  important  nuclei 
hidden  in  the  same  female  flower  are  exactly  alike  in 
hereditary  constitution,  and  so  are  the  two  generative 
nuclei  derived  from  a single  pollen  grain.  In  conse- 
quence of  this  fact,  the  observed  character  of  the  endo- 
sperm may  be  regarded  as  a true  guide  to  the  nature 
of  the  plant  into  which  the  associated  embryo  will 
afterwards  develop.  The  hereditary  qualities  of  the 
two  are  exactly  the  same. 

It  is  not  difficult  to  find  a variety  of  Indian  corn  in 
which  the  endosperm  is  yellow,  and  another  in  which 
the  colour  of  this  tissue  is  white,  owing  to  the  absence 
of  any  visible  yellow  pigment.  If  a female  flower  of  a 
white  variety  is  fertilized  with  pollen  taken  from  a 
yellow  variety,  the  resulting  grain  shows  its  hybrid 
nature  by  the  presence  of  the  yellow  colour  in  its  endo- 
sperm. This  is  found  to  be  a regular  rule.  Grains 
upon  a plant  of  a white  strain  which  has  been  pollinated 
with  ‘ white  pollen  ’ are  white,  but  if  pollinated  from 


INHERITANCE  IN  MAIZE 


179 


a yellow  strain  the  grains  are  yellow.  On  the  other 
hand,  the  grains  upon  a plant  belonging  to  a yellow 
strain  retain  their  yellow  colour  even  if  the  flowers 
which  produce  them  have  been  pollinated  from  a white 
variety. 

These  facts  are  expressed  in  technical  language  by 
saying  that  yellowness  is  dominant  over  whiteness,  and 
the  latter  is  said  to  be  recessive. 

Let  us  now  suppose  that  we  have  sown  a number 
of  the  yellow  grains  derived  from  the  cross  yellow  x 
white*  or  white  x yellow,  and  that  we  have  exposed 
the  female  flowers  of  the  resulting  plants  at  the  proper 
stage  of  their  existence  to  the  influence  of  pollen 
derived  from  a pure  white  strain,  taking  care  that 
none  of  their  own  hybrid  pollen  falls  upon  them  at 
the  same  time.  The  result  of  this  experiment  takes 
us  at  once  to  the  very  heart  of  the  Mendelian  theory. 
Half  the  total  number  of  grains  obtained  in  this  way 
— from  the  cross  {white  x yellow)  x white — are  white, 
and  half  are  yellow. 

Thus  in  an  experiment  carried  out  in  the  manner 
described  there  were  obtained  upon  ninety-five  plants  : 

Yellow  grains  26,792,  or  50'03  per  cent. 

White  grains  26,751,  „ 49-97  „ 

But  we  must  go  further  than  this.  On  sowing  the 
white  grains  obtained  in  this  second  generation  (F2), 
and  allowing  the  plants  obtained  from  them  mutually 
to  pollinate  one  another,  cobs  were  obtained  bearing 
exclusively  white  grains  without  any  trace  of  yellow- 
ness. 

♦ X is  to  be  read  ‘ fertilized  with  pollen  from.’ 

12 — 2 


i8o 


MENDELISM 


Half  the  grains,  then,  of  the  parentage  {white  x 
yellow)  X white  are  pure  white  in  colour,  and  not  to  be 
distinguished  from  grains  of  the  parentage  white  x white 
even  after  an  extensive  examination  of  their  offspring, 
which  is  the  most  rigorous  test  we  are  able  to  apply. 

The  yellow  grains  born  upon  the  same  hybrid  plants 
(F2)*  had  clearly  each  of  them  one  white  parent — 
namely,  the  plant  from  which  the  white  pollen  was 
derived.  On  sowing  these  yellow  grains  and  once 
more  pollinating  by  pure  white,  a precisely  similar 
result  was  observed  to  that  obtained  in  the  preceding 
generation — that  is  to  say,  these  plants,  derived  from 
yellow  grains,  produced  once  more  50  per  cent,  of 
white  grains  and  50  per  cent,  of  yellow.  We  are, 
therefore,  led  to  suppose  that  the  yellow  grains  born 
upon  the  hybrid  plants  are  of  precisely  the  same  nature 
as  the  original  yellow  hybrid  grains  {white  x yellow), 
since  their  behaviour  when  pollinated  from  the  same 
white  strain  is  identical.  We  may  express  the  result 
so  far  obtained  in  the  form  of  the  following  diagram  : 
P*  white  X yellow 

Fi  yellow  x white 


F2  white  X yellow  (50  %)  (50  %)  white  x white 


Fg  white  (50  %)  yellow  (50  %)  white  (exclusively). 

* The  following  shorthand  expressions  are  adopted  to  denote 
the  different  generations  in  cross-breeding  experiments  : P is 
the  generation  of  the  original  parents  ; is  the  first  genera- 
tion of  offspring — the  cross-bred  seeds  and  the  plants  to 
which  they  give  rise.  To  the  Fg  generation  belong  the  seeds 
produced  upon  the  F^  plants,  and  the  plants  to  which  they 
give  rise,  and  so  on. 


INHERITANCE  IN  MAIZE  i8i 

The  pollen  of  the  Fj  plants  {i.e.,  those  plants  which 
were  derived  from  the  yellow  cross-bred  grains) — when 
applied  to  the  female  flowers  of  the  same  pure  white 
strain  of  maize,  caused  in  like  manner  the  appearance 
of  white  and  yellow  grains  in  equal  numbers.  This 
result  is  equally  well  expressed  by  the  above  diagram 
on  simply  regarding  the  yellow  in  as  the  male  parent 
(pollen-parent)  instead  of  as  the  female  parent  (seed- 
parent)  of  Fg. 

What  is,  then,  the  meaning  of  these  results  ? The 
case  is  really  very  simple.  The  germ-cells  (ova  and 
pollen-nuclei)  of  the  cross-bred  plants  {white  x yellow) 
must  be  potentially  either  pure  white  or  pure  yellow, 
with  no  blending  of  these  characters.  Further,  the 
two  kinds  (yellow  and  white)  of  male  germ-cells  or 
pollen-nuclei  must  arise  in  equal  numbers,  and  the 
same  must  be  true  of  the  female  germ-cells  or  ova. 
By  this  supposition  only  can  the  observed  facts  be 
explained.  If  the  supposition  is  true,  then,  when  the 
cross-bred  plant  (Fj)  is  crossed  again  with  the  pure 
white  form,  its  white  germ-cells  give  rise  to  white 
grains  which  are  of  the  nature  {white  x white),  and 
are  therefore  pure.  Its  yellow  germ-cells  give  rise  to 
yellow  grains  which  are  of  the  nature  {yellow  x white). 
And,  since  the  number  of  yellow-  and  white-bearing 
germ-cells  is  equal,  the  number  of  yellow  and  of  white 
grains  produced  in  this  way  is  approximately  the  same. 
The  yellow  grains  are  of  the  same  composition  as  the 
original  cross-bred  grains  obtained  by  crossing  pure 
white  with  pure  yellow,  and  we  have  seen  that  they 
behave  in  exactly  the  same  way  on  further  cross- 


i82 


MENDELISM 


breeding.  This  conclusion  is  at  least  so  far  firmly 
established  that  no  alternative  hypothesis  has  been 
put  forward  which  will  explain  the  facts. 

We  have  next  to  consider  what  will  be  the  result  of 
crossing  our  cross-bred  plants  with  one  another  instead 
of  with  the  pure  white  form.  The  following  possi- 
bilities present  themselves  : 

A yellow  female  gamete  may  pair  with  a yellow  male  gamete. 

)>  )>  >»  »i  >»  white  I, 

A white  „ „ „ „ yellow  „ 

))  » >»  n » white  „ 

All  these  combinations  are  equally  likely  to  occur, 
because  in  each  plant  there  are  the  same  number  of 
yellow  and  white  female  gametes  as  well  as  of  yellow 
and  white  male  gametes.  In  the  long-run,  therefore, 
each  of  the  above  pairings  will  be  found  to  have  taken 
place  in  an  equal  number  of  cases.  The  grains  which 
we  shall  obtain,  then,  will  be  yellow  and  white  in  colour, 
and  the  two  kinds  will  occur  in  the  following  propor- 
tions : I pure  white  ; 2 white  x yellow  or  yellow  x 
white,  which,  as  we  have  already  seen,  will  be  yellow 
in  appearance  ; and  i pure  yellow.  Altogether,  we 
shall  expect  a ratio  of  3 yellow  grains  to  i white. 

In  an  actual  experiment  the  following  result  was 
obtained  : 

Yellow  grains  16,592,  or  74‘5  per  cent. 
tYhite  „ 5j68i,  „ 25  5 ,, 

— ^that  is  to  say,  a ratio  of  2’9  yellow  to  i white. 

The  expression  lA  : 2 Aa : la,  in  which  A represents 
the  dominant  character  (yellow)  and  a the  recessive 
character  (white),  may  be  spoken  of  as  a Mendelian 


MONO-HYBRID  CROSSES 


183 


formula.  It  indicates  the  proportion  in  which  the 
two  pure  types  and  their  hybrid  brethren  will  appear, 
on  breeding  together  the  offspring  of  a simple  or 
mono-hybrid  cross — i.e.,  one  in  which  attention  is 
paid  to  the  behaviour  of  a single  pair  of  characters  only. 

So  far  we  have  been  dealing  with  a pair  of  characters 
consisting  in  the  presence  and  absence  respectively  of 
a particular  pigment.  Precisely  similar  results  are  to 
be  obtained  in  the  case  of  a pair  of  structural  char- 
acters. The  endosperm,  or  reserve  substance,  of  cer- 
tain varieties  of  Indian  corn  shows  a smooth  surface, 
and  contains  an  essentially  starchy  reserve  material, 
whilst  in  other  races  the  surface  of  the  endosperm  is 
wrinkled  and  the  reserve  product  is  of  a sugary  nature. 
This  sugary  endosperm  is  characteristic  of  the  kinds 
of  corn  largely  used  in  the  United  States  of  America 
as  a table  vegetable. 

On  crossing  together  a variety  with  smooth  starchy 
grains  and  one  with  wrinkled  sugary  grains,  the  grains 
immediately  resulting  are  smooth  and  starchy,  no 
matter  whether  the  starchy  strain  is  used  as  the  seed- 
parent  or  as  the  pollen-parent — that  is  to  say,  thestarchy 
character  is  dominant,  a dominant  character  being  one 
which  appears  in  Fj  to  the  complete  or  almost  com- 
plete exclusion  of  the  corresponding  character  ex- 
hibited by  the  other  parent,  which  is  spoken  of  as 
recessive.  In  the  present  case  the  sugary  character  is 
recessive. 

The  further  behaviour  of  the  cross  between  smooth 
and  wrinkled  is  precisely  the  same  as  that  of  yellow 


i84 


MENDELISM 


crossed  with  white.  Thus,  if  the  hybrid  plants  are  bred 
together  or  self-fertilized,  the  resulting  cobs  will  exhibit 
a proportion  of  three  smooth  grains  to  one  wrinkled 
grain.  In  an  actual  example  there  were  obtained  5,310 
smooth  grains  and  1,765  wrinkled,  or  75-06  per  cent, 
of  the  former  and  24-94  cent,  of  the  latter. 

In  a further  generation  the  wrinkled  grains  breed 
true.  On  the  average  one  out  of  every  three  smooth 
grains  does  the  like.  The  remaining  two  smooth  grains 
are  of  hybrid  nature,  and  on  self-fertilization  yield 
again  the  same  proportion  of  three  smooth  to  one 
wrinkled.  Such  hybrid  grains  and  the  plants  into 
which  they  develop  are  spoken  of  as  heterozygotes. 

Thus,  if  we  write  B for  smooth  and  b for  wrinkled, 
the  following  scheme  will  express  the  result  of  crossing 
together  plants  which  bear  these  characters,  and  after- 
wards self-fertilizing  the  offspring  obtained  j 

7 P (dominant  character)  B x b (recessive  character) 


Bb 


(heterozygote  looking  like  B) 
on  self-fertilization  yields 


trT  Fj  BB 


Bb  bB 


bb 

(extracted 

recessive) 


(extracted 

dominant) 


(heterozygotes  looking  like  BB) 
on  self-fertilization  each  gives 


BB  BB  ' Bb  bB 

I civf  v'o  ^ 


(extracted  ^ 

dominant)  and  so  on 


(extracted 

recessive) 


bb  bb 


and  so  on,  and  so  on, 
breeding  breeding 
true.  true. 


BB  BB 


bb  bb 

and  so  on,  and  so  on, 
breeding  breeding 
true.  true. 


DI-HYBRID  CROSSES 


185 


So  far  we  have  seen  that  both  a pair  of  structural 
characters  and  a pair  of  colour  characters  can 
‘Mendehze,’  according  to  the  phrase  coined  by  the 
Germans — that  is  to  say,  the  germinal  representa- 
tives of^Juch  pairs  of  characters  remain  perfectly 
distinct  in  the  hybrid  plant,  and  separate  completely 
at  the  formation  of  its  gametes,  in  such  a way  that 
an  equal  number  of  gametes  arises  containing  either 
character^ 

The  members  of  a pair  of  characters  which  behave 
in  this  way  on  crossing  are  called  allelomorphs.  When 
a pair  of  gametes  fuse  together  in  the  process  of  fer- 
tilization the  resulting  cell  is  known  as  a zygote.  A 
zygote  formed  by  the  conjunction  of  two  like  gametes 
is  called  a homozygote.  When  the  gametes  contain 
opposite  members  of  a pair  of  allelomorphs  the  result 
is  called  a heterozygote.  The  same  terms  may  also  be 
applied  to  the  adult  multicellular  organisms  into  which 
these  fertilized  egg-cells  develop. 

We  have  still  to  consider  what  happens  when  parents 
are  crossed  which  differ  in  more  than  one  pair  of  allelo- 
morphs. The  actual  result  is  as  follows  : 

Suppose  a smooth  yellow  tj^pe  of  maize  to  be  crossed 
with  a wrinkled  white  variety,  both  smoothness  and  ' 
yellowness  being  dominant.  The  grains  produced  in 
Fj  are  therefore  yellow  and  smooth.  Let  the  Fj  plants, 
arising  from  the  smooth  yellow  heterozygote  grains, 
be  crossed  with  the  wrinkled  white  parent,  which 
is  recessive  in  respect  of  both  these  characters. 
In  this  way  the  true  nature  of  every  germ  cell 
produced  by  the  heterozygote  will  be  able  to  manifest 


i86 


MENDELISM 


itself  in  the  visible  character  of  the  grain  produced 
from  it. 

The  following  result  was  actually  obtained  in  this 
way  ; 

Smooth  yellow  grains  2,869,  or  25*3  per  cent. 

Smooth  white  grains  2,933,  or  257  „ 

Wrinkled  yellow  grains  2,798,  or  24*5  „ 

Wrinkled  white  grains  2,803,  or  24-5  „ 

Thus  we  see  that  a nearly  equal  number  of  the  germ 
cells  of  the  double  heterozygote  bears  each  of  the  four 
possible  combinations  of  characters — that  is  to  say,  it 
is  an  even  chance  whether  a particular  gamete,  which 
bears  the  allelomorph  yellowness,  bears  also  smoothness 
or  wrinkledness.  In  other  words,  the  two  pairs  of 
allelomorphs  segregate  in  entire  independence  the  one 
of  the  other.  It  is  particularly  to  be  noticed  that  we 
arrive  in  this  way  at  two  perfectly  new  combinations 
of  characters,  which  were  not  shown  by  the  original 
parent  strains.  We  have  synthesized  two  new  sorts 
of  maize  with  smooth  white  and  wrinkled  yellow  grains 
respectively.  In  a precisely  similar  way,  if  the  cross 
is  made  between  strams  of  which  the  grains  are  re- 
spectively smooth  white  and  wrinkled  yellow,  we 
should  obtain  in  F2  the  new  combinations  smooth 
yellow  and  wrinkled  white. 

The  result  obtained  on  self-fertilizing  the  hybrid 
plant  is  somewhat  more  complicated. 

If  we  write  A for  yellowness,  a for  whiteness,  B for 
smoothness,  and  h for  wrinkledness  as  before,  AB  x ab 
gives  the  heterozygote  AJB&b-  Equal  numbers  of  the 
germ  cells  of  the  heterozygote  will  be  of  the  composi- 
tions AB,  Ah,  aB,  and  ah. 


Fig.  14. — Mendeltan  Proportions  in  Maize. 

Cobs  born  by  heterozygote  plants  pollinated  with  the  recessive,  showing 
equality  of  smooth  and  wrinkled  and  of  coloured  and  white  grains. 


\To  face  p.  18$. 


THE  LIBRARY 
Of  THE 

UNIVERSITY  OF  ILLINOIS 


DI-HYBRTD  CROSSES  187 

AU  the  following  zygotic  combinations  are,  then, 
equally  likely  5 


ABAB 

ABAb 

ABaB 

A Bab 

AbAB 

AbAb 

AbaB 

A bob 

aBAB 

aBAb 

aBaB 

aBab 

abAB 

abAb 

abaB 

abab 

Altogether  there  are  sixteen  combinations.  The 
result  can  be  expressed  more  shortly  in  the  form 
(A  +2Aa  + a)  (B  + 2Bh  + h),*  which  will  be  found  to  give 
the  above  terms  when  expanded.  Thus  the  combina- 
tion of  the  Mendelian  formulae  for  Fg  when  each  of 
the  pairs  of  allelomorphs  is  considered  separately, 
gives  us  the  formula  for  the  two  pairs  of  allelomorphs 
considered  simultaneously. 

The  same  result  may  also  be  written  in  the  form  ; 


4^  ] , . 1 

r iB 

2 Bb 

I b 

8 Aa  r combined  with  i 

2 B 

4Bb 

2 b 

4 a J ' 

1 

2 Bb 

I b 

or 


AB  2 ABb  Ab 

2 A ab  4 AaBb  2 Aab 

aB  2 aBb  ab* 

* It  is  customary  to  condense  these  expressions  as  far  as 
possible  by  never  repeating  the  same  letter  more  than  once 
in  each  term.  Thus,  A stands  for  AA,  B for  BB,  and  so  on. 
On  expansion,  i,e,y  multiplying  together  the  contents  of  the  two 
brackets,  ^ xi5  gives  A BAB,  AxBb  gives  ABAb,  and  so  on 
for  all  the  other  terms  of  the  expression. 


i88 


MENDELISM 


Let  us  consider  the  external  appearance  of  these 
various  types  in  the  particular  example  before  us. 

Nine  of  the  above  sixteen  terms  include  A and  B, 
and' are  therefore  smooth  yellow  in  appearance.  (We 
need  not  stop  to  consider  whether  a or  6 or  both  are 
present  in  addition,  since  these  are  recessive.) 

Three  terms  include  A and  b,  B being  absent.  These, 
therefore,  appear  wrinkled  yellow. 

Three  include  a and  B,  A being  absent.  These, 
therefore,  appear  smooth  white. 

One  contains  a and  b only,  and  is,  therefore,  wrinkled 
white. 

With  regard  to  internal  constitution  : 

The  nine  individuals  of  appearance  AB  include  the 
following  types : 

One  pure,  ABAB,  breeding  true  to  the  smooth 
yellow  type  on  self-fertilization. 

Two  ABAb,  heterozygous  in  respect  of  the 
pair  B-b,  but  pure  yellow. 

Two  ABaB,  heterozygous  in  respect  of  A-a, 
but  pure  smooth. 

Four  A Bab,  heterozygous  in  respect  of  both 
pairs  of  characters. 

The  three  individuals  of  appearance  Ab  include  the 
following  types  : 

One  pure,  AbAb,  breeding  true  to  the  (new) 
wrinkled  yellow  type. 

Two  Abab,  giving  both  wrinkled  yellow  and 
wrinkled  white. 


DI-HYBRID  CROSSES  189 

The  three  individuals  of  appearance  aB  include  the 
following  types  : 

One  pure,  aBaB,  breeding  true  to  the  (new) 
smooth  white  type. 

Two  aBab,  giving  both  smooth  white  and 
wrinkled  white. 

The  remaining  individual  is  ah  in  appearance  and 
abab  in  constitution,  and  breeds  true  to  the  wrinkled 
white  type. 

The  expected  behaviour  of  all  these  different  types 
can  be  followed  out  by  the  aid  of  suitable  breeding 
experiments,  and  not  only  has  this  been  done  in  the 
case  of  the  cross  which  we  have  been  considering,  but 
precisely  similar  phenomena  have  been  shown  to  be 
taking  place  for  a large  number  of  other  characters  in 
many  different  species  of  plants  and  in  a good  many 
animals  as  well. 

We  are  now  in  a position  to  state  the  important 
proposition  known  as  Mendel’s  law,  which  is  to  the 
following  effect  : 

The  gametes  of  a heterozygote  bear  the  pure  parental 
allelomorphs  completely  separated  from  one  another, 
and  the  numerical  distribution  of  the  separate  allelo- 
morphs in  the  gametes  is  such  that  all  possible  com- 
binations of  them  are  presfent  in  approximately  equal 
numbers.  (Note  that  it  is  impossible  for  more  than 
one  member  of  the  same  pair  of  allelomorphs  to  occur 
together  in  the  same  gamete.) 

This  is  the  essence  of  the  great  discovery  made  by 
Gregor  Mendel,  Abbot  of  Brunn,  and  published  by 


MENDELISM 


190 

him  in  the  Transactions  of  the  Brunn  Natural  History 
Society  in  1866.  By  some  extraordinary  chance 
Mendel’s  paper  was  entirely  lost  sight  of  until  the 
same  facts  were  independently  rediscovered  in  1899 
by  de  Vries  working  in  Holland,  by  Correns  in  Germany, 
and  by  Tschermak  in  Austria. 

Gregor  Johann  M6ndel  was  born  on  July  22,  1822, 
at  Heinzendorf,  near  Odrau,  in  Austrian  Silesia.  In 
1843  he  entered  as  a novice  the  Augustine  Convent  at 
Altbrunn,  and  was  ordained  priest  in  1847. 

Mendel  was  a teacher  of  natural  science  in  the  Brunn 
Realschule  from  1853  to  1868,  when  he  was  appointed 
Abbot  of  his  monastery.  During  this  time  he  was 
largely  occupied  with  experiments  in  cross-breeding  a 
great  variety  of  plants,  and  some  idea  of  his  activity 
in  this  line  of  scientific  work  is  to  be  gathered  from  a 
perusal  of  his  letters  to  the  German  biologist  Nageli, 
a correspondence  which  has  recently  been  published 
by  Professor  Correns.  Mendel  himself  only  published 
the  result  of  his  work  with  peas,  and  that  of  a few  of 
his  experiments  with  Hieracium. 

After  1873  the  cares  associated  v/ith  the  position  of 
Abbot  of  Brunn  appear  to  have  prevented  further 
biological  work.  His  death  took  place  in  1884,  two 
years  after  that  of  Charles  Darwin,  to  whom  Mendel 
was  thirteen  years  junior. 

Mendel’s  own  experiments — that  is  to  say,  the 
chief  ones  published  by  him — ^were  made  with  peas,  a 
kind  of  plants  which  were  found  to  be  remarkably 
well  suited  to  this  kind  of  work.  Seven  pairs  of 
characters  in  these  plants  were  found  to  behave  in 


GREGOR  MENDEL 


191 

precisely  the  same  manner  as  those  characters  of  the 
maize-plant  which  have  been  already  described,  and 
in  all  of  them  the  phenomenon  of  dominance  also 
appeared.  The  characters  dealt  with  by  Mendel  were 
as  follows,  the  dominant  member  of  the  pair  being  in 
each  case  placed  first  : 

Smooth  seeds,  and  wrinkled  seeds. 

Yellow,  and  green  reserve  material — t.e.,  cotyledons. 

Deeply  coloured  (grey),  and  nearly  colourless  test  as 
or  seed-coats. 

Inflated  or  stiff,  and  wrinkled  or  soft  pods. 

Green,  and  yellow  pods. 

Flowers  scattered  up  the  stem,  and  flowers  in  a 
terminal  bunch  or  umbel. 

Tall,  and  dwarf  stems. 

As  the  result  of  these  experiments  Mendel  came  to 
the  conclusion  with  which  his  name  is  now  closely 
associated — ^that  the  male  and  female  germ-cells  of 
hybrid  plants  contain  each  of  them  one  or  the  other 
member  only  of  any  pair  of  differentiating  characters 
exhibited  by  the  parents,  and  that  each  member  of 
such  a pair  of  characters  is  represented  in  an  equal 
number  of  germ-cells  of  both  sexes.  Furthermore, 
separate  pairs  of  differentiating  characters  (allelo- 
morphs) conform  to  this  law  in  complete  independence 
of  one  another. 

Although  in  Mendel’s  own  experiments  one  member 
of  each  pair  of  differentiating  characters  was  always 
dominant,  dominance  is  by  no  means  an  universal 
phenomenon  when  different  varieties  of  plants  are 
crossed  together.  In  a considerable  number  of  in- 


193 


MENDELISxM 


stances  the  heterozygote  is  found  to  exhibit  an  appear- 
ance which  is  more  or  less  intermediate  between  the 
types  of  character  shown  by  the  parents.  It  may  be 
almost  exactly  intermediate,  or  the  appearance  of  the 
cross-bred  form  may  be  nearer  to  that  of  one  parent 
than  to  that  of  the  other.  Dominance  is  clearly  only 
an  extreme  case  of  this  latter  phenomenon.  The 
term  ‘ dominance  ’ is  applied  to  those  cases  in  which 
the  appearance  of  the  hybrid  offspring  is  so  near  to 
that  of  one  parent  as  to  be  no  longer  clearly  distin- 
guishable from  it. 

In  other  cases,  still  of  a simple  Mendelian  nature, 
the  appearance  of  the  heterozygote  may  be  quite 
different  from  that  of  either  parent  homozygote. 
An  excellent  example  of  this  nature  is  afforded  by  the 
Andalusian  fowls  studied  by  Messrs.  Bateson  and 
Punnett.  And  this  will  also  serve  as  our  first  illus- 
tration of  the  application  of  these  principles  to  animals 
as  well  as  to  plants.  The  facts  of  the  case  are  as 
follows : 

The  ‘ blue  ’ type  of  Andalusian  appears  to  be  a 
heterozygote  form  which  has  never  been  got  to  breed 
true.  When  a pair  of  these  birds  are  mated  together 
only  about  half  their  offspring  are  like  themselves,  the 
remainder  being  entirely  different.  Half  these  re- 
maining ‘ wasters  ’ are  black,  and  half  are  nearly 
white,  showing  only  a few  black  ‘ splashes.’  If,  now, 
a pair  of  the  black  wasters  are  mated  together,  they 
breed  perfectly  true,  yielding  only  black  offspring  like 
themselves.  Similarly  the  splashed  whites  mated 
together  give  rise  to  splashed  white,  and  nothing  else. 


HETEROZYGOTE  FORMS 


193 


Both  these  forms,  then,  the  black  and  the  splashed 
white,  are  clearly  pure  homozygotes.  On  mating  a 
black  and  a splashed  white  together,  black-bearing 
gametes  and  white-bearing  gametes  will  meet  together 
in  fertilization.  In  every  case  in  which  this  form  of 
mating  was  carried  out  the  resulting  chicks  were 
invariably  blue. 

The  gametes  of  the  blue  type  of  Andalusians,  then, 
according  to  our  supposition,  do  not  bear  the  blue 
character  at  aU,  Half  of  them  contain  the  black  and 
half  of  them  the  splashed  white  allelomorph.  Such 
gametes,  meeting  by  chance  when  a pair  of  blue  An- 
dalusians are  mated  together,  give  rise  to  the  zygotes 
— one  black-black,  two  black-white,  one  white-white — 
the  black-whites  being,  of  course,  blue  in  appearance 
as  before. 

Now,  we  may  put  this  explanation  to  the  test  by  a 
very  simple  experiment — namely,  by  mating  the  sup- 
posed heterozygote  blues  with  the  black  and  with 
the  splashed  white  types  respectively.  Both  these 
forms  of  mating  were  examined  by  Bateson  and 
Punnett,  and  the  results  were  as  follows  : It  was  found 
that  blues  crossed  with  blacks  gave  rise  to  equal 
numbers  of  blue  and  of  black  offspring,  whilst  when 
blues  were  crossed  with  splashed  whites  there  ap- 
peared blue  and  splashed  white  chicks  in  equal  numbers. 
And  by  a repetition  of  the  process  it  could  be  shown 
that  the  blues  so  obtained  were  heterozygotes  as  before. 
Here,  then,  we  have  clear  evidence  that  equal  numbers 
of  the  germ-cells  produced  by  the  blue  birds  bear  the 
pure  black  allelomorph  and  the  pure  splashed  white 

13 


194 


MENDELISM 


allelomorph  respectively,  since  half  the  offspring  ob- 
tained on  mating  the  blue  birds  with  black  are  black, 
and  half  the  offspring  obtained  on  mating  them  with 
splashed  white  are  splashed  white.  The  following 
scheme  of  inheritance  illustrates  the  phenomena 
described  s 


I (zygote) 

blue  heterozygote 

I (gametes) 

black  and  white 

1 

2 (zygote) 

1 1 

1 black-black  2 black-white 

1 

I white-white 

1 

(blue  heterozygote) 

1 

3 (gametes) 

black 

black 

white 

black 

1 

white 

1 

white 

3 (zygote) 

1 

black 

1 1 1.  I 

I black  2 blue  i white  white 

3 (gametes) 

black 

1 

/ white 

black 

’ 1 

\white 

4 (zygote) 

1 

all  blue  heterozygotes 

A case  which  is  closely  similar  to  that  of  the  An- 
dalusian fowl  is  afforded  by  the  cross  between  Primula 
sinensis  and  Primula  stellata. 

P.  sinensis  crossed  with  P.  stellata  gives  rise  to  a 
type  which  is  different  from  either  parent,  being  in 
some  respects  intermediate  between  the  two.  The 
hybrid  is  so  distinct  that  a special  name  has  been  given 
to  it,  and  the  new  type  is  known  as  P.  fyramiialis. 
So  far  it  has  been  found  impossible  to  obtain  a strain 
of  P.  Pyramidalis  which  will  breed  true.  Oh  self- 
fertilization  the  offspring  are  found  to  show  the  types 


Fig.  15. — Primula  Sinensis  crossed  with  P.  Stellata. 

Above,  the  parents.  In  the  middle,  the  heterozygote  offspring 
— P.  pyramidalis.  Below,  the  result  of  self-pollinating  P.  pyramidalis : 
I.  P.  sinensis  ; 2,  P.  pyramidalis : i.  P,  stellata. 


[To  face  p.  194* 


THE  LIBRARY 
OF  THE 

UHIYERSITY  OF  ILLIHOIS 


MENDELISM 


195 

of  P.  sinensis,  P.  fyramidalis,  and  P.  stellata  in  the 
ratio  of  I : 2 : I. 

Cases  like  the  above  illustrate  the  essential  part  of 
Mendel’s  law  even  better  than  those  in  which  domi- 
nance is  present,  the  characteristic  proportion  of  one 
of  each  homozygote  type  to  two  .of  the  heterozygote 
being  at  once  recognisable  in  such  a case  without  the 
necessity  for  further  breeding ; whereas,  in  cases  where 
there  is  dominance,  further  study  is  necessary  in  order 
to  distinguish,  among  the  individuals  of  dominant 
appearance,  those  which  are  pure  dominant  and  those 
which  are  heterozygous  in  constitution. 

In  concluding  our  account  of  the  simpler  forms  of 
Mendelian  phenomena  we  may  consider  one  further 
point  with  regard  to  the  nature  of  the  two  allelomorphs 
making  up  any  particular  pair.  In  what  is  probably 
a majority  of  the  cases  hitherto  examined  the  dominant 
and  recessive  allelomorph  seem  to  represent  respec- 
tively the  presence  and  absence  of  something.  Thus 
the  dominance  of  colour  to  absence  of  colour,  or  white- 
ness, is  a very  frequent  phenomenon.  And  in  some  of 
the  more  complex  cases  to  be  described  in  the  next 
chapter  we  shall  find  the  presence  and  absence  of  a 
particular  factor  very  often  behaving  as  a pair  of  Men- 
delian allelomorphs.  The  question  arises  as  to  how 
far  this  conception  should  be  extended.  It  seems,  for 
instance,  somewhat  far-fetched  to  speak  of  dwarfness 
as  being  simply  determined  by  the  absence  of  the  factor 
for  tallness,  though  it  is  not  impossible  that  this  may 
be  the  correct  way  of  looking  at  the  facts.  Be  this  as 
it  may,  it  is  to  be  remembered  that  a Mendelian  pair 

13—2 


196 


MENDELISM 


often  represents  the  presence  and  absence  respectively 
of  a particular  feature. 

BIBLIOGRAPHY. 

Mendel,  G.  J.  : Versuche  ueber  Pflanzenhybriden,  1865. 

„ Ueber  einige  aus  kunstlicher  Befruchtung 

gewonnen  Hieracium-Bastarde,  1869. 
PuNNETT,  R.  C.:  Mendelism,  fifth  edition,  1919. 

Bateson,  W.  : Mendel’s  Principles  of  Heredity,  1902  and  1909. 
„ Address  to  the  Zoological  Section,  British 

Association,  1904. 

Lock,  R.  H.  : In  Annate  of  the  Royal  Botanic  Gardens 
Peradeniya,  1904. 


CHAPTER  VIII 


MENDELISM  {continued) 

Mendel’s  law,  as  stated  in  the  preceding  chapter, 
has  already  been  found  to  hold  good  in  a very  large 
number  of  cases — cases  in  which  all  kinds  of  characters 
are  concerned,  belonging  to  many  different  species  of 
animals  and  plants.  In  certain  instances,  however, 
complications  arise,  and  these  may  be  treated  of  in 
two  main  sections. 

The  first  kind  of  complication  arises  from  the  pheno- 
menon known  as  coupling.  The  essence  of  this  pheno- 
menon consists  in  the  existence  of  some  kind  of  affinity 
occurring  in  the  same  individual  between  allelomorphs 
which  belong  to  distinct  pairs.  In  consequence  of  such 
an  affinity  exceptions  are  found  to  the  rule  that  sepa- 
rate pairs  of  allelomorphs  segregate  independently. 

The  closeness  of  the  connection  between  the  char- 
acters concerned  shows  a series  of  gradations  in  dif- 
ferent cases.  In  the  simplest  cases  of  all,  what  are 
loosely  spoken  of  as  separate  characters  are  found  on 
closer  examination  to  be  only  different  aspects  of  one 
and  the  same  characteristic  feature.  These  cases,  then, 
offer  no  real  exception  to  the  rule,  for  only  one  pair  of 
allelomorphs  is  actually  concerned.  As  an  example,  we 
may  take  the  case  of  the  wrinkled  sugary  type  of  maize 
already  contrasted  with  the  smooth  starchy  variety. 
The  essential  difference  between  the  two  kinds  depends 

upon  the  fact  that  in  the  former  the  reserve  product 

197 


MENDELISM 


198 

laid  down  in  the  endosperm  is  different,  being  largely 
of  a sugary  nature  instead  of  being  starchy.  With 
this  circumstance  is  associated  the  presence  of  a larger 
proportion  of  water  in  the  unripe  grain.  And  the 
result  of  this  is  that,  when  the  grain  dries,  its  surface 
falls  into  folds.  The  sugary  nature  of  the  grains  also 
causes  them  to  take  on  a more  hyaline  or  semi- 
transparent appearance  than  the  grains  of  the  starchy 
variety.  All  these  characters,  if  they  can  be  so  called, 
behave  on  crossing  as  a single  Mendelian  allelomorph, 
and  are  doubtless  represented  in  the  germ  cells  by  a 
single  substantive  representative. 

A simple  example  of  what  may  probably  be  regarded 
as  a real  case  of  coupling  is  afforded  by  certain  colour 
characters  exhibited  by  pea-plants.  In  these  plants 
coloured  flowers,  a red  or  purple  colouration  in  the 
axils  of  the  leaves,  and  a marked  pigmentation  of  the 
testas,  or  seed-coats,  are  always  associated  together  on 
the  same  plants ; so  that,  if  we  find  a plant  which  has 
green  leaf  axils,  we  may  be  sure  that  its  flowers  will 
be  white,  and  the  testas  of  its  seeds  only  slightly  pig- 
mented. On  crossing  plants  bearing  coloured  axils, 
coloured  flowers,  and  pigmented  testas,  on  the  one 
hand,  with  plants  bearing  green  axils,  white  flowers, 
and  unpigmented  seed-coats,  on  the  other,  the  two  sets 
of  characters  are  found  to  behave  as  a simple  pair  of 
allelomorphs,  and  the  simultaneous  appearance  of 
colour  in  these  different  situations  doubtless  depends 
upon  the  presence  of  a particular  pigment  in  the  plant 
which  exhibits  it.  Nevertheless,  we  can  scarcely 
fail  to  look  upon  these  three  separate  manifestations 
of  the  pigment  as  representing  distinct  characters,  and 


COUPLING 


199 


this  being  so,  we  suppose  their  germinal  representa- 
tives to  be  coupled  together  in  such  a way  that  they 
remain  associated  at  the  time  when,  during  the  forma- 
tion of  the  germ-cells  of  the  heterozygote,  other  allelo- 
morphs become  independently  segregated. 

And  this  way  of  looking  at  the  facts  is  further 
justified  by  the  behaviour  of  the  characters  in  ques- 
tion in  another  species  of  plant.  For  in  the  sweet  pea 
it  is  possible  for  the  coupling  between  these  characters 
to  be  broken  down,  so  that  a plant  which  exhibits 
green  leaf  axils  may,  under  certain  circumstances,  bear 
coloured  flowers.  In  such  a plant  the  leaf-axU-colour 
and  the  flower-colour  must  clearly  be  represented  by 
independent  allelomorphs. 

In  other  cases,  again,  there  may  be  coupling  between 
characters  which  have  no  obvious  relation  to  one 
another  at  all.  In  illustration  we  may  take  the  case 
of  a cross  between  two  strains  of  peas,  one  of  which  had 
white  flowers  and  opened  its  buds  several  days  earlier 
than  the  second,  the  blossoms  of  which  were  purple. 

The  Fj  plants  (with  purple  blossoms)  came  into  flower 
at  a period  intermediate  between  those  of  the  parents. 
In  Fj  506  plants  were  grown  successfully.  Some  of 
these  flowered  as  early  as  the  white  parent,  and  others 
as  late  as  the  purple  parent ; but  the  majority  of  the 
plants  ranged  between  these  two  extremes,  so  that  it 
was  impossible  to  rank  the  individuals  into  definite 
classes  in  respect  of  so  indefinite  a character  as  time 
of  flowering.  On  making  a perfectly  arbitrary  division, 
however,  it  was  found  that  175  purple  and  104  white 
plants  were  in  flower  on  a certain  day,  and  that  208 
purple  and  19  white  plants  did  not  open  their  buds 


200 


MENDELISM 


until  afterwards.  There  is,  therefore,  clearly  some 
coupling  between  the  presence  of  white  blossoms  and 
early  flowering  on  the  one  hand,  and  between  lateness 
and  purple  flowers  on  the  other.  Two  characters 
more  diverse  than  colour  of  the  flowers  and  time  of 
flowering  could  at  first  sight  scarcely  be  imagined. 

In  this  last  example  the  two  characters  concerned 
do  not  appear  to  be  completely,  but  only  partially 
coupled.  This  phenomenon  of  partial  gametic  coupling 
was  discovered  by  Bateson  and  Punnett  in  the  Sweet 
Pea,  and  has  since  been  observed  in  a number  of  plants 
and  in  several  animals.  [In  animals,  by  far  the  most 
completely  known  case  is  that  of  the  Fruit-fly  {Droso- 
phila), investigated  by  T.  H.  Morgan  and  his  school, 
see  p.  270.]  In  the  Sweet  Pea,  one  of  the  first  cases 
to  be  investigated  was  concerned  with  the  shape  of  the 
pollen  grains,  whether  oval  or  round,  and  the  colour 
of  the  flowers,  whether  blue  or  red. 

When  blue-oval  is  crossed  with  red-round,  the 
Fi  plants  are  all  blue-oval.  But  in  F2,  instead  of 
getting  9 blue-oval,  3 blue-round,  3 red-oval,  and 
I red-round,  the  numbers  of  the  different  kinds  of 
plants  obtained  closely  approached  the  following 
proportion — 177  : 15  : 15  : 49. 

Such  a series  would  be  produced  if  the  allelomorphs 
concerned  were  associated  in  the  gametes  in  the 
following  proportion:  7 blue-oval,  i blue-round, 
I red-oval,  7 red-round,  as  may  easily  be  verified  by 
multiplication.  Enormous  numbers  of  plants  must 
naturally  be  examined  before  it  can  be  asserted  that 
the  series  actually  chosen  is  really  the  correct  one ; in 
fact,  mere  statistics  are  hardly  capable  of  proving  so 


COUPLING 


201 


complicated  a proportion  as  this  in  the  absence  of 
independent  considerations. 

[In  the  case  of  other  characters,  the  coupling  between 
members  of  distinct  pairs  was  closer,  and  suggested 
gametic  ratios  approaching  15:1:1:15  or  31 : i : i : 31, 
so  that  Bateson  and  Punnett  have  supposed  that  the 
sum  of  the  series  is  always  an  exact  power  of  2 
(8,  16,  64,  etc.),  and  that  the  phenomenon  is  due  to  a 
differential  division  of  a germ-cell  followed  by  more 
divisions  of  one  of  the  daughter  cells  than  of  the  other. 
If,  for  example,  a primitive  germ-cell  containing  all 
four  factors — those  for  oval  and  round  pollen,  blue  and 
red  flowers^ — divides  twice  in  such  a way  that  the  four 
cells  produced  contain  respectively  the  factors  blue 
and  oval,  blue  and  round,  red  and  oval,  red  and  round, 
and  then  the  cells  bearing  blue-oval  and  red-round 
divide,  while  the  others  do  not,  the  observed  excess  of 
germ-cells  bearing  these  characters  will  be  accounted 
for.  But  Professor  Morgan  in  Drosophila  does  not  find 
these  ratios  of  7 : i,  15  : i,  etc.,  and  has  offered  an  en- 
tirely different  explanation  of  the  facts  which  will  be 
mentioned  in  the  chapter  on  Cytology  (Chapter  IX.). 

It  should  be  noticed  that  characters  which  are 
coupled  are  always  introduced  into  the  double  hetero- 
zygote from  the  same  parent.  If,  in  the  Sweet  Pea, 
one  parent  is  blue-oval,  the  other  red-round,  the  blue 
flower  will  be  coupled  with  the  oval  pollen  and  the  red 
flow'er  with  the  round  pollen  in  the  grandchildren. 
But  if  such  characters  are  introduced  from  different 
parents,  the  converse  phenomenon  of  gametic  repulsion 
is  seen.  If  a blue-flowered  round-pollened  plant  is 


203 


MENDELISM 


crossed  with  a red  oval-pollened  plant,  the  blue, 
instead  of  being  coupled  with  the  oval  pollen  in  the 
gametes  of  the  heterozygote,  is  ‘ repelled  ’ by  it,  and 
the  great  majority  of  the  gametes  will  bear  the  blue- 
round  or  red-oval  characters.  This  is  obviously  only 
another  form  of  coupling,  in  which  a dominant  char- 
acter is  coupled  with  a recessive  instead  of  two 
dominants  being  coupled  together,  and  the  general 
law  of  coupling  (or  as  the  Americans  call  it,  linkage) 
may  be  expressed  thus:  In  the  case  of  characters 
which  show  gametic  coupling,  those  factors  which  are 
associated  together  in  the  homozygotes  used  as  parents 
tend  to  be  associated  together  in  the  gametes  produced 
by  the  double  (or  multiple)  heterozygote  offspring. 
The  question  of  coupling  of  Mendelian  characters  with 
a sex-determining  factor  (‘  sex-hmited  inheritance  ’) 
will  be  mentioned  in  Chapter  IX.] 

The  second  class  of  complications  that  we  have  to 
deal  with — although  the  term  complication  may  be 
to  a certain  extent  justified  in  connection  with  it — 
does  not  involve  any  exception  to  Mendel’s  law  of 
segregation.  The  phenomenon  of  so-called  reversion 
on  crossing  has  long  been  familiar  to  biologists.  Its 
meaning,  however,  was  totally  obscure,  and  even  the 
Mendelian  was  at  first  unable  to  offer  any  explanation. 
The  phenomenon  consists  in  the  appearance,  in  the 
offspring  of  a cross,  of  a character  which  was  not 
visibly  present  in  either  parent,  and  in  many  cases  this 
character  can  properly  be  regarded  as  ancestral — it  is 
a character  which  has  been  lost  by  both  parents  in 
the  course  of  their  divergent  evolution  from  a common 


MASKED  CHARACTERS 


203 


primitive  form.  Now,  these  cases  differ  entirely  from 
those  of  the  appearance  of  a heterozygote  form  on 
crossing,  such  as  are  due  to  the  combined  action  of  the 
two  parental  allelomorphs  in  the  cross-bred  offspring, 
because  in  true  cases  of  reversion  a certain  proportion 
of  the  reversionary  individuals  of  Fg  are  found  to  breed 
true,  which  a simple  heterozygote  will  never  do. 

It  has  been  found  that  the  essential  part  of  this 
phenomenon  of  reversion  on  crossing  consists  in  the 
existence  in  the  parents  of  certain  hereditary  factors — 
allelomorphs,  in  fact — ^which,  although  by  themselves 
invisible,  yet,  when  combined  in  cross-breeding  with 
certain  other  allelomorphs,  belonging  to  independent 
pairs,  lead  to  the  appearance  of  new  visible  characters. 

The  term  reversion  cannot  properly  be  applied  to 
these  phenomena  as  a class,  because,  in  the  first  place, 
characters  may  arise  in  this  way  which  cannot  be 
regarded  as  ancestral,  and,  secondly,  because  reversions 
may  take  place  in  other  ways ; for  example,  the 
reappearance  of  a simple  recessive  character  would 
legitimately  be  ranked  among  reversions.  The  best 
general  name  for  the  class  of  phenomena  we  are  about 
to  describe  is  perhaps  masking  of  characters,  or  cryfto- 
merism,  the  latter  being  the  term  employed  by  Tscher- 
mak,  who  was  the  first  to  describe  these  phenomena 
in  connection  with  Mendelian  ratios. 

In  the  simpler  cases  an  invisible  or  masked  factor 
derived  from  one  parent,  on  becoming  associated  with 
a different  factor  born  by  the  other  parent,  and  already 
visibly  represented  among  the  external  features  of  this 
second  parent,  maJces  itself  apparent  among  the  visible 
characteristics  of  the  heterozygote.  In  such  a case 


204 


MENDELISM 


the  characteristic  appearance  exhibited  by  the  hetero- 
zygote may  subsequently  become  permanent,  owing 
to  the  building  up  of  a type  which  is  a homozygote  in 
respect  of  both  the  necessary  factors. 

This  may  be  made  clearer  by  a definite  illustration. 

A pea-plant  characterized  by  the  presence  of  a greyish 
or  brownish  testa  to  its  seeds  (grey)  was  crossed  with 
a plant  having  nearly  colourless  testas  (white).  The 
testas  of  the  Fj  plants  were  marked  with  bright  purple 
dots  on  a grey  ground  (purplel.  These  hybrid  plants 
were  self-pollinated,  and  in  Eg  the  three  types  appeared 
in  the  following  proportions  : 9 pm  pie,  3 grey,  4 white. 
What  is  the  meaning  of  this  ratio  ? In  order  to 
complete  the  ordinary  expectation  for  a simple  Men- 
delian  case  in  which  two  pairs  of  allelomorphs  are 
concerned  (di-hybridism)  we  must  write  down  the 
following  expression  : 

( purple  \ / no  purple  \ / purple  \ . j / no  purple/ 

" \ grey  / ' ^ I grey  / ‘ \ no  grey  / ‘ \ no  grey  J 


But  it  would  seem  that  the  purple  character  cannot 
appear  when  the  grey  colour,  or  some  factor  con- 
stantly associated  with  this  colour,  is  absent,  as  is 
the  case  in  the  original  white  parent  from  which  the 
factor  for  purple  spots  was  derived.  Consequently,  the 

three  / 1 plants  are  indistinguishable  from  the 

(no  greyj 

/no  P'lrplel  Qj.  vyhites,  and  we  thus  arrive  at 

I no  grey  J 

the  result  which  was  described  as  being  the  one  actually 
obtained — namely,  9 purple  : 3 grey  ; 4 white. 

In  other  respects  this  example  is  precisely  like  the 


MASKED  CHARACTERS 


205 


case  of  two  pairs  of  allelomorphs  described  on  p.  187. 
We  may  write  A foi  presence  of  grey  pigment,  a for 
absence  of  grey  pigment,  B for  presence  of  purple,  and 
b for  its  absence.  Then  the  original  cross  was  of  the 
form  Ab  xaB,  from  which  AaBb  resulted  in  F,.  And 
the  visible  characters  of  the  types  which  appeared 
in  Fj  would  be  represented  by  gAB ^Ab + {3aB 
lab).  On  referring  to  the  account  given  on  p.  187  it 
will  be  seen  that  one  in  nine  of  the  purple  plants  is  of 
the  constitution  ABAB,  and  may  be  expected  to 
breed  true. 

A precisely  similar  result  may  be  obtained  in  Fj  in 
cases  where  there  is  no  reversion  in  Fj.  In  the 
following  example  a white  pea,  which  did  not  contain 
tire  masked  purple  factor,  was  crossed  with  a ' maple- 
seeded  ’ pea.  The  characteristic  feature  of  maple  is  a 
marbling  of  brown  spots  on  a grey  ground  colour.  In 
Fj  the  marbling  was  dominant,  and  the  seeds  resembled 
the  maple  parent. 

In  Fj  there  appeared  9 maple  : 3 grey  : 4 white — 
i.e.,  the  same  ratio  as  in  the  previous  case,  this  time 
without  reversion.  This  ratio  is  brought  about  by  the 
simple  combinations  of  two  pairs  of  allelomorphs  A-a, 
and  C-c,  C being  unable  to  manifest  itself  unless  A is 
present  in  the  same  zygote.  As  a matter  of  fact,  in 
this  particular  case  C does  sometimes  just  manage 
to  appear  in  the  absence  of  A,  the  result  being  a 
white  seed  with  a sort  of  faint  ‘ ghost  ’ of  a maple 
marking. 

When  a strain  bearing  both  maple  marking  and 
purple  spots  is  crossed  with  a white  in  which  neither 
of  these  factors  is  latent,  we  cam  easily  calculate  tire 


206 


MENDELISM 


ratio  to  be  expected  in  Fa  by  using  the  formula 
(A+2Aa  + a)  (B  + 2Bb  + b)  (C  + 2Cc  + c).  The  result 
works  out  as  follows  (writing  m for  maple,  p for  purple, 
and  g for  grey)::  ^jmpg,  gmg,  gpg,  3g,  (gmp,  3m, 
3p,  iw).  Since  g is  absent  from  all  the  members  of 
the  series  enclosed  in  the  bracket,  these  appear  white, 
or  nearly  so,  the  total  number  of  whites  being  thus  16. 
And  the  numbers  obtained  in  an  actual  experiment 
accorded  closely  with  the  expected  ratio  27 ; 9 : g : 3 : 16. 

Among  the  sixteen  whites,  some  wiU  be  bearing  the 
factors  for  m and  p,  others  that  for  p only,  others  that 
for  m only,  whilst  one  in  sixteen  will  contain  neither  of 
these  factors.  Until  such  invisible  differences  between 
the  different  white  plants  are  actually  proved  to  be 
present  the  whole  account  so  far  given  will  remain  more 
or  less  hypothetical.  The  proof  is  obtainable  by  cross- 
ing the  different  whites  with  a pure  grey  strain.  The 
grey  factor  being  thus  introduced,  the  whites  which 
contain  a />  or  an  w factor  will  exhibit  the  same  in 
their  offspring.  A number  of  the  whites  obtained  in 
Fa  and  in  later  generations  were  actually  crossed  with 
the  same  grey-seeded  plant.  Some  of  the  offspring 
showed  both  the  maple  and  the  purple  character, 
others  the  maple  without  the  purple,  others  the  purple 
without  the  maple,  and  others,  again,  showed  neither ; 
the  seeds  of  these  last  being  exactly  like  those  of  the 
grey  parent  owing  to  simple  dominance  of  the  grey 
allelomorph  over  white. 

The  first  example  of  this  kind  of  phenomenon  to  be 
observed  in  the  case  of  animals  was  one  described  by  the 
French  zoologist  Cu6not  [in  mice].  Cu6not’s  original 
account  has  had  to  be  somewhat  modified  in  view 


EPISTATIC  CHARACTERS  207 

of  more  recent  work  by  Miss  Durham,  and  her 
account  of  the  facts  runs  as  follows.  For  the  sake  of 
simplicity  we  shall  deal  in  the  first  instance  with 
only  four  types  of  colour  [in  mice]— the  ‘agouti’  or 
wild  grey  colour,  black,  * chocolate,’  and  albino.  The 
behaviour  of  these  colours  in  heredity  can  be  described 
in  terms  of  three  pairs  of  allelomorphs  : 

Gg : The  presence  and  absence  of  the  factor  which 
gives  the  ‘ agouti  ’ or  grey  pattern  in  the  hairs. 

Bb : Presence  and  absence  of  the  black  determiner. 

Cc  : Presence  and  absence  of  colour. 

Where  C is  present  without  G or  B,  the  colour  is 
chocolate,  the  proper  formula  for  such  an  animal 
being  CCggbb  or  Ccggbb.  Black  mice  may  be  CCggBB, 
etc.,  and  grey  mice  CCGGBB,  etc. 

All  albino  mice  are  to  be  represented  as  those  from 
which  C — i.e.,  the  chocolate  colour — is  absent ; but 
either  G or  B,  or  both,  may  be  present  (but  masked) 
in  an  albino  individual. 

When  B and  C are  both  present,  the  colour  is  black, 
and  not  chocolate.  We  cannot,  however,  speak  of 
black  as  being  dominant  to  chocolate,  since  these  two 
factors  belong  to  independent  allelomorphic  pairs.  A 
new  term  is  therefore  required  for  this  relationship, 
and  also  for  the  relationship  between  grey  and  black. 
Bateson’s  suggestion  for  the  required  terminology  may 
be  given  in  his  own  words  : ‘ We  can,  perhaps,  best 
express  the  relation  between  the  grey  and  the  black 
by  the  use  of  the  metaphor  " higher  and  lower,”  and 
I therefore  suggest  the  term  epistatic  as  applicable  to 
characters  which  have  to  be,  as  it  were,  lifted  oS  in 
order  to  allow  the  lower  or  hypostatic  character  to 


2o8 


MENDELISM 


appear.’  Thus  grey  is  epistatic  to  black,  and  black 
is  epistatic  to  chocolate. 

A curious  phenomenon  appears  in  the  case  of  yellow 
mice,  which  must  be  briefly  mentioned  here  on  account 
of  its  bearing  upon  a subject  discussed  in  the  next 
chapter.  Yellow  appears  to  be  epistatic  to  grey  as 
well  as  to  black,  but  yellow  mice,  so  far  as  the  evidence 
goes,  are  always  heterozygous.  Cu6not’s  experiment 
to  demonstrate  this  fact  was  as  follows : 

When  YyGGCC  is  crossed  with  yyGGCC,  equal 
numbers  of  yellow  and, grey  offspring  are  to  be  ex- 
pected, since  G is  hypostatic  to  Y.  In  various  crosses 
of  this  nature  Cuenot  actually  obtained  177  yellows 
and  178  greys,  from  which  we  may  deduce  that  the 
heterozygote  yellow  was  giving  off  the  expected  pro- 
portion of  gametes  bearing  the  yellow  character  {i.e., 
50  per  cent.). 

When  such  heterozygous  yellows  are  bred  together, 
the  expected  result  would  be  as  follows  ; 

YyGGCC  X YyGGCC  - YYGGCC  + 2 YyGGCC  + yyGGCC 

' . ' 

3 yellow  I grey 

Eighty-one  yellow  mice  were  actually  obtained  in 
this  way.  Among  them  some  twenty-seven  would 
naturally  be  expected  to  be  pure  dominant,  and  to  give 
yellow  only  when  crossed  with  black  or  grey  indi- 
viduals. To  Cuenot’s  astonishment,  he  found  on  making 
the  necessary  crosses  that  every  one  of  these  eighty-one 
yellows  gave  some  black  or  grey  among  its  offspring ; 
not  one  of  them  was  a pure  homozygous  yellow. 

The  facts  are  explicable  in  one  of  two  ways.  On  the 


YELLOW  MICE 


209 


earlier  hypothesis  there  was  supposed  to  be  some 
obstacle  in  the  way  of  the  fertilization  of  a yellow- 
bearing egg  by  a spermatozoon  bearing  the  same 
character.  In  this  case,  since  spermatozoa  are  in 
great  numerical  excess  as  compared  with  eggs,  it  is 
still  possible  for  every  Y-bearing  egg  to  be  fertilized 
by  a spermatozoon  lacking  Y,  as  well  as  half  the  non- 
yellow eggs  by  Y-bearing  sperms,  giving  a ratio  of 
3 ; I in  jF2  from  yellow  by  yellow.  On  the  other 
hand,  it  is  possible  that  pure  yellow  zygotes  are  formed 
in  fertilization,  but  for  some  unknown  cause  are  incap- 
able of  development.  In  this  case  a third  of  the  yellow 
progeny  would  be  wanting,  and  the  expected  ratio  would 
be  2 : I.  Cu6not’s  figures  led  to  the  belief  that  the  first 
hypothesis  was  the  correct  one,  but  in  further  experi- 
ments by  Castle  and  by  Miss  Durham  the  ratio  was 
found  to  approach  2 : i in  a majority  of  cases.* 

We  have  still  to  describe  a case  in  which  two  latent 
factors,  one  derived  from  each  parent,  give  rise,  by 
their  simultaneous  presence  in  the  zygote  produced, 
to  the  appearance  of  an  entirely  new  character.  The 
following  example  is  the  first  one  of  the  kind  to  be 
completely  elucidated,  and  is  one  of  those  studied  by 
Messrs.  Bateson  and  Punnett  and  Miss  Saunders. 

The  white-flowered  variety  of  sweet-pea  known  as 
Emily  Henderson  was  found  to  exist  in  two  forms, 
only  to  be  distinguished  from  one  another  by  the 
shape  of  the  pollen  grains  which  they  produced.  In 
one  of  the  two  the  shape  of  the  pollen  is  elliptical 

* [It  is  now  definitely  established  that  homozygous  yellow  zygotes 
are  formed,  but  die  during  embryonic  life.  Disintegrating  embryos 
have  been  found  in  the  uterus.] 


14 


210 


MENDELISM 


(long  pollen),  in  the  other  it  is  approximately  spherical 
(round  pollen).  Sweet-peas  normally  undergo  self- 
poUination,  so  that  the  two  types  naturally  remain 
distinct.  Let  us  see  what  happened  when  the  long- 
and  the  round-pollened  forms  were  crossed  together. 

The  cross-bred  plants  (Fj)  had  coloured  flowers — ■ 
flowers  of  the  old  - fas'hioned  purple  type  known  to 
florists  as  Purple  Invincible,  which  is  characterized  by 
a purple  standard  and  blue  wings.  The  pollen  pro- 
duced by  these  plants  was  of  the  long  type.  Thus  as 
regards  the  shape  of  the  pollen  grains  there  was  simple 
dominance.  But  the  union  of  two  white-flowered 
types  has  given  rise  to  a series  of  plants  all  possessing 
a definite  colour  character — purple  with  blue  wings. 
This  character  is  very  probably  the  same  as  that 
exhibited  by  the  common  ancestor  of  all  our  cultivated 
sweet-peas.  Here,  then,  we  seem  to  have  a clear  case 
of  reversion  to  the  ancestral  type  on  crossing.  We 
shall  find  that  the  Mendelian  principles  will  enable  us 
to  arrive  at  a dear  conception  of  the  mechanism  of 
this  process. 

The  cross-bred  plants  were  self-pollinated,  and  in  Fj 
the  following  types  made  their  appearance  in  approxi- 
mately the  proportions  given  • 


Purple  Invincible 

3' 

Picotee  •••  ••• 

27J 

1 1 1 ) 

Painted  Lady 

27] 

I , 

Tinged  white  ... 

White 

9] 

112 

1 u 

LATENT  CHARACTERS 


211 


Painted  Lady  is  a well-known  colour  type  which  is 
characterized  by  the  presence  of  a red  standard  and 
white  wings.  Picotee  and  tinged  white  are  also  forms 
well  known  to  the  sweet-pea  fancy.  They  appear  to 
be  diluted  forms  of  the  purple  and  Painted  Lady  types 
respectively,  their  appearance  depending  upon  the 
presence  of  a definite  diluting  factor  in  addition  to  the 
factor  for  the  colour  in  question,  or  perhaps  more 
properly  upon  the  absence  of  the  proper  strengthening 
factor  which  converts  Picotee  into  purple,  and  tinged 
white  into  Painted  Lady. 

The  following  explanation  of  the  result  so  far 
described  has  now  been  well  established  by  further 
experiment.  In  the  first  place,  we  may  consider  all  the 
coloured  forms  together  as  a single  group  opposed  to 
white.  It  is  now  clear  that  the  coloured  type  of  Fj  is 
due  to  the  meeting  together  of  two  factors,  one  of  them 
borne  by  one  white  parent  and  the  other  by  the  second 
and  it  is  necessary  for  both  these  factors  to  be  simul- 
taneously present  in  order  that  any  colour  may  make 
its  appearance.  We  may  call  these  two  factors  C and 
R,  denoting  the  absence  of  either  by  c and  r respectively. 
By  the  simple  Mendelian  behaviour  of  these  two  pairs 
of  factors  C-c  and  R-r,  the  ratio  of  nine  coloured 
plants  to  seven  white  appearing  in  Fa  is  readily 
explicable,  and  the  way  in  which  this  happens  is  shown 
in  the  diagram  (Fig.  i6). 

To  explain  the  presence  of  the  four  different  types  of 
coloured  plants  which  make  their  appearance  in  Fg, 
two  further  pairs  of  allelomorphs  are  called  in.  The 

dominant  member  (B)  of  one  of  these,  when  present 

14 — 3 


212 


MENDELISM 


in  combination  with  C and  R,  produces  the  purple 
or  Picotee  colour  (blue),  whilst  its  absence  (6)  in  pre- 
sence of  C and  R is  accompanied  by  the  appearance 
of  the  red  colours — Painted  Lady  and  tinged  white. 

Purple  Invincible  and  Painted  Lady  are  regarded  as 
intensified  forms  of  Picotee  and  tinged  white  respec- 
tively. The  presence  of  the  second  factor  {T)  is  attended 
by  the  development  of  the  full  colours  purple  and 


CR 


cR 


Cr 


c r 


The  shaded  squares  represent  coloured  plants,  the  blank  squares 
white  plants. 

Painted  Lady  ; its  absence  it)  causes  the  appearance  of 
the  diluted  forms  Picotee  and  tinged  white. 

B and  T may  be  present  when  either  C or  i?  or  both 
are  absent ; the  resulting  plant  has  then  white  flowers. 
And  it  is  interesting  to  notice  that  the  ultimate  recessive 
white,  containing  c r h t,  occurs  only  once  among  256 
individuals  of  F^.  The  whole  apparently  complex  system 
of  floral  colours  is  thus  explained  by  the  simple  Men- 
delian  behaviour  of  four  separate  pairs  of  allelomorphs. 
Bateson  and  his  collaborators  have,  therefore,  provided 


§miM 

rI 

pa 

llil 

0 0 

i 

fm. 

cr 

cR 

Cr 

C r 

cr 

Cr 

llpr 

Cr 

cr 

cr 

cr 

Fig  16. 


MENDELIAN  CHARACTERS 


213 


a complete  account  of  the  phenomenon  of  reversion 
on  crossing,  an  account  which  has  already  been  demon- 
strated to  hold  good  in  other  instances  besides  that 
of  the  sweet-pea.  The  facts  are  expressed  in  the 
following  manner  by  their  discoverers  r ‘ “ Reversion  ” 
is  thus  seen  to  be  a simple  and  orderly  phenomenon 
due  to  the  meeting  of  factors  belonging  to  distinct 
though  complementary  allelomorphic  pairs,  which  at 
some  moment  in  the  phylogeny  of  the  varieties  have 
each  lost  their  complement.’* 

Dr.  G.  H.  Shull  has  classified  the  different  known 
types  of  latency  in  the  following  manner : 

1.  Latency  due  to  separation.  In  this  form  of 
latency  an  allelomorph,  when  acting  alone,  has  no 
external  manifestation,  and  is  only  rendered  visible 
by  combination  with  another  allelomorph  belonging 
to  a distinct  pair.  In  this  way  such  ratios  as  9 : 3 : 4 
or  9 : 7 may  arise.  We  have  already  observed  an 
example  of  this  phenomenon  in  the  case  of  the  two 
factors  required  to  produce  colour  in  the  sweet-pea. 

2.  Rare  cases  have  been  described  of  latency  due 
to  combination.  This,  instead  of  being  due  to  separa- 
tion, is  the  result  of  a union  in  the  same  zygote  of  two 
dominant  allelomorphs,  either  of  which  alone  will 
produce  a manifest  character.  When  both  allelo- 
morphs are  present  the  character  fails  to  appear. 

3.  Latency  due  to  hypostasis,  or  the  masking  of 
one  dominant  character  by  another  character  also 
dominant  and  epistatic  to  the  masked  character,  has 
already  been  described  in  the  case  of  the  coat  colours 

♦ Proceedings  of  the  Royal  Society,  B.  vol.  77,  p.  238. 


214  MENDELISM 

of  mice.  The  simplest  typical  ratio  thus  produced  is 

12  : 3 : I. 

4.  The  last  type  of  latency  described  by  Dr.  Shull 
is  of  a totally  different  kind.  It  is  represented  by  the 
phenomenon  to  which  the  term  ‘ latency ' wa's  origin- 
ally applied  by  de  Vries.  This  latency  consists  in  the 
disappearance  of  certain  characters  under  the  influence 
of  poor  nutrition,  or  other  changes  of  conditions. 
Examples  of  this  phenomenon  described  in  the  chapter 
on  Mutation  were  the  submerged  and  floating  leaves 
of  the  water-ranunculus,  and  the  red  and  white  colour 
of  the  flowers  of  a species  of  primula. 

We  may  now  proceed  to  pass  in  rapid  review  a 
selection  of  the  more  remarkable  instances  of  Mendel ian 
inheritance  which  have  been  so  far  demonstrated. 

The  ease  with  which  characteristics  of  colour  can  be 
distinguished  and  defined  has  naturally  led  to  a good 
deal  of  attention  being  paid  to  the  phenomena  of  their 
inheritance.  In  this  way  many  cases  of  simple  domi- 
nance have  been  discovered  in  plants  and  in  animals, 
as  well  as  several  examples  of  reversion  in  Fj,  followed 
in  both  cases  by  a Mendelian  segregation  of  characters. 

Thus  the  colours  of  many  flowers  afford  perfectly 
simple  phenomena,  whilst  other  cases,  like  the  sweet- 
peas  and  the  closely  similar  case  of  stocks  studied  by 
Miss  Saunders,  have  required  long  and  arduous  ex- 
periment for  their  elucidation.  No  case  of  this  kind 
hitherto  examined  has  been  definitely  proved  to  be 
non-Mendelian. 

Colour  characters  which  follow  Mendel’s  law  have 
been  observed  in  mice,  rats,  rabbits,  guinea-pigs. 


MENDELIAN  CHARACTERS 


215 


pigeons,  fowls,  cats,  and  so  on.  In  butterflies  and 
other  insects,  and  even  in  snails,  similar  phenomena 
have  been  described.  The  study  of  the  larger  domestic 
animals  awaits  for  the  present  the  proper  endowment 
of  these  researches.  When  this  takes  place,  the 
inheritance  of  far  more  important  characters  than 
colour  will  be  adequately  studied  to  the  great  profit  of 
all  who  are  concerned  in  the  breeding  industry.  Hurst 
has  already  shown  from  an  examination  of  the  stud- 
book that  the  bay  and  brown  colours  of  thoroughbred 
horses  are  Mendelian  dominants  to  chestnut.* 

Other  characters  of  the  most  diverse  kinds  are  also 
similarly  inherited.  We  have  already  referred  to 
structural  characters  in  maize  and  in  peas.  Stature 
is  a character  which  is  definitely  inherited  in  many 
plants.  Among  more  subtle  characters  a similar  mode 
of  transmission  has  been  fomid  in  the  case  of  differences 
in  chemical  composition,  and  in  that  of  immunity 
from  and  susceptibility  to  the  attacks  of  certain 
diseases.  The  thrum-eyed  condition  of  the  primrose 
has  been  shown  by  Bateson  and  Gregory  to  be  a 
Mendelian  dominant  to  the  pin-eyed  condition,  so  that 
we  have  here  a partial  solution  of  a problem  which  is 
rendered  specially  interesting  from  the  fact  that  it 
baffled  Darwin. 

A study  of  numerous  pedigrees  has  enabled  Bateson 
and  others  to  show  that  in  the  case  of  the  human  race 
certain  congenital  diseases  are  simply  transmitted  from 
parent  to  offspring  in  accordance  with  Mendel’s  law. 

* [Other  colour  characters  have  now  been  shown  to  have 
Mendelian  inheritance  in  horses,  cattle,  sheep,  etc.] 


2i6 


MENDELISM 


How  far  the  influence  of  the  Mendelian  principles 
may  extend  we  do  not  yet  know.  But  it  is  certain  that 
very  few,  if  any,  cases  have  so  far  been  discovered  in 
which  differentiating  characters  do  not  behave  in  this 
way  when  the  types  which  exhibit  them  are  crossed 
together.  Experiments  have  now  been  made  upon  a 
great  variety  of  plants  and  animals,  involving  a con- 
siderable diversity  of  kinds  of  characters.  Nevertheless 
it  is  scarcely  possible  to  cite  a case  in  which  it  is 
definitely  and  certainly  known  that  Mendel’s  law, 
subject  to  the  modifications  already  described,  does 
not  hold  good.  Cases  of  various  kinds  are,  indeed, 
recorded,  but  these  records  are  derived  from  experi- 
ments either  carried  out  before  the  bearings  of  the 
Mendelian  phenomena  were  at  all  fully  appreciated,  or 
— ^and  this  is  the  most  frequent  case — without  any 
knowledge  at  all  of  Mendci^’s  discovery. 

Thus  a considerable  number  of  cases  were  formerly 
described  in  which  the  first  cross  or  heterozygote  of  Fj 
bred  true  instead  of  segregating  in  F*.  There  is  some 
doubt  whether  any  case  of  this  kind  will  really  stand 
criticism ; Millardet’s  case,  for  example,  which  was 
mentioned  at  the  end  of  the  last  chapter  but  one,  has 
never  been  confirmed.  It  is  quite  certain  that  among 
all  the  numerous  crosses  studied  during  the  last  six 
years  no  example  of  the  kind  has  been  substantiated. 
The  most  recent  cases  to  be  described  of  a first  cross 
breeding  true  are  those  of  de  Vries,  and  at  these  we 
are  bound  to  pause,  because  de  Vries  is  surpassed  by 
no  recent  observer  in  weight  of  authority.  Neverthe- 
less, de  Vries’  cases  are  of  so  complex  a kind  that  we 


MENDELIAN  CHARACTERS  217 

have  some  hesitation  in  accepting  them  without  further 
study. 

It  must  be  admitted  that  the  suggestion  that  Men- 
delian  segregation  may  be  a universal  phenomenon 
accompanying  the  formation  of  the  germ-cells  of  all 
animals  and  plants  is  one  which  will  be  disputed  by 
many  biologists.  There  is  ample  evidence,  they  will 
say,  of  first  crosses  breeding  true  in  the  case  of 
numerous  plant  and  animal  hybrids.  But  there  are 
seyeral  points  to  be  considered  before  we  can  accept 
the  mere  fact  of  a uniform  progeny  as  sufficient  evidence 
that  there  was  nothing  in  the  nature  of  segregation 
amongst  the  gametes  of  the  hybrid. 

To  prove  a negative  is  proverbially  difficult,  and 
the  attempt  to  show  that  hybrid  characters  exist 
which  do  not  segregate  in  the  germ  cells  of  the  hybrid 
is  no  exception  to  this  rule.  In  favour  of  the  opposite 
contention  the  following  considerations  may  be 
alleged : 

The  characters  for  which  non-segregation  has  been 
asserted  are  generally  complex  and  difficult  to  define. 
Such  characters,  for  instance,  are  the  general  habit 
of  a plant  or  the  general  shape  of  a flower.  Characters 
such  as  these  may  reasonably  be  supposed  to  owe 
their  appearance  to  the  interaction  of  a considerable 
number  of  independent  allelomorphs. 

Where  two  such  factors  are  concerned,  in  cases 
where  the  heterozygote  form  is  intermediate,  we 
should  expect  one  out  of  sixteen  Fa  individuals  to 
resemble  each  parent  form,  the  remaining  fourteen 
plants  being  more  or  less  intermediate. 


2i8 


MENDELISM 


With  three  such  factors  we  should  expect  only  one 
of  each  parent  type  to  appear  among  sixty-four 
individuals. 

There  seems  to  be  no  reason  for  doubting  that  the 
total  number  of  visible  factors,  even,  which  go  to 
make  up  the  total  shape  of  a flower  may  be  consider- 
ably greater  .than  this ; and  the  number  of  invisible 
allelomorphs  upon  which  these  depend  may  be  pro- 
portionately more  numerous  still.  In  such  a case,  in 
an  experiment  where  the  total  number  of  offspring 
grown  was  limited,  types  like  the  original  parents 
might  never  be  seen,  simply  because  the  sample  taken 
was  not  sufficiently  large. 

Again,  we  have  to  consider  the  possibility  of  a 
differential  fertility  among  the  various  allelomorphic 
combinations.  This  might  lead  to  an  intermediate 
form,  actually  breeding  true  in  spite  of  the  fact  that 
segregation  was  going  on  in  its  germ  - cells.  The 
possibility  has  to  be  borne  in  mind  that  among  the 
offspring  of  two  widely  different  species  those  indi- 
viduals which  more  nearly  resemble  their  immediate 
hybrid  parent  may  have  a better  chance  of  survival 
than  the  forms  which  have  more  characters  in  common 
with  a single  pure  grandparent.  The  only  piece  of  evi- 
dence we  have  bearing  upon  this  point  is  the  fact  that  we 
know  the  hybrid  form  itself  to  be  capable  of  surviving. 

De  Vries  has  published  a very  interesting  observa- 
tion, which  ought  to  be  mentioned  in  this  connec- 
tion. It  appears  that  two  forms  of  (Enothera,  which 
originated  in  a cross,  and  are  distinguished  as 
Lcsia  and  Velutina  respectively,  possess  female  gametes 


NORMAL  FERTILIZATION 


219 


half  of  which  can  be  shown  to  bear  the  tall  and  half 
the  dwarf  character.  All  the  functional  pollen  grains 
of  L^ta  bear  the  tall  character,  and  all  the  functional 
pollen  grains  of  Velutina  bear  the  dwarf  character. 
De  Vries  does  not  offer  the  obvious  suggestion  that 
half  of  the  total  number  of  pollen  grains  in  each  case 
are  impotent.*  If  this  suggestion  should  prove  to 
represent  the  truth,  OBnothera  Iceta  would  afford  a 
case  of  a hybrid  which  breeds  true  in  spite  of  the  fact 
that  typical  segregation  is  taking  place  among  its 
germ-cells.  In  the  light  of  this  discovery  it  is  clear 
that  non-segregation  cannot  properly  be  asserted  in 
any  given  case  until  the  hybrid  has  been  crossed  with 
each  of  the  parent  forms  on  a considerable  scale. 

We  may  now  turn  for  a brief  space  to  some  of  the 
cases  in  which  we  have  as  yet  no  certain  knowledge  of 
the  manner  in  which  inheritance  proceeds. 

The  most  obvious  extension  of  Mendel’s  law  to 
processes  where  it  cannot  be  directly  shown  to  hold 
good  is  to  suppose  that  the  same  rule  applies  to  cases 
of  normal  fertilization  as  to  hybrid  fertilizations.  We 
should  then  picture  the  former  process  as  taking 
place  in  somewhat  the  following  way.  Every  visible 
character  of  the  individual  which  can  be  separately 
distinguished,  and  which  on  cross-breeding  would  be 
inherited  on  ordinary  Mendelian  lines,  must  be  repre- 
sented in  the  gametes  by  a definite  factor  of  some  kind, 

*[In  an  analogous  case  in  Drosophila  it  seems  certain  that  the 
explanation  is  of  this  kind.  See  H.  J.  Muller  ‘Genetic  Variability, 
Twin  Hybrids  and  Constant  Hybrids,  in  a case  of  balanced  lethal 
factors.’  Genetics,  vol.  iii.,  1918,  p.  422-499.] 


220 


MENDELISM 


possibly  by  a definite  substance  or  combination  of  sub- 
stances. The  pair  of  parental  factors  for  a particular 
character  would  combine  on  fertilization,  and  at  the 
formation  of  the  gametes  in  the  offspring  its  members 
would  separate  as  perfectly  definite  entities,  to  re- 
combine when  these  gametes  meet  once  more  with  their 
corresponding  mates.  Such  a definite  segregation  of 
characters  taking  place  within  a pure  strain  would  be 
very  difficult  of  absolute  demonstration,  but  it  is  hard  to 
avoid  the  conclusion  that  this  is  a true  deduction  from 
the  facts  observed  when  cross-breeding  takes  place. 
Such  a segregation  would  formerly  have  been  thought  a 
very  small  assumption  in  comparison  with  that  of  the 
segregation  of  pairs  of  allelomorphs  of  which  no  trace 
is  externally  visible,  and  yet  the  latter  assumption 
has  now  been  shown  to  be  perfectly  well  established. 

This  idea  of  unit  characters,  capable  of  being  inherited 
independently  of  one  another,  is  one  of  the  most 
important  conceptions  which  has  ever  been  introduced 
into  the  science  of  biology,  and  the  introduction  of  it 
has  followed  as  the  direct  result  of  Mendel’s  work.  It 
is  a conception  which  has  led  to  a complete  change  in 
our  ideas  of  heredity,  since  we  no  longer  look  upon 
the  individual  as  a unit,  but  find  ourselves  compelled 
to  study  separately  the  independent  characters  of 
which  the  individual  is  built  up.  The  idea  of  the 
individual  as  a living  mosaic — ^an  idea  put  forward  long 
ago  by  Naudin  with  only  a partial  realization  of  its 
significance — has  thus  returned  to  us.  In  this  con- 
nection a curious  problem  presents  itself.  What 
would  be  left  if  we  could  imagine  all  the  separable 


ESTABLISHMENT  OF  NEW  SPECIES  221 


characters  of  a living  creature  as  having  been  taken 
away  ? Would  there,  or  would  there  not,  be  any 
residuum  ? Upon  this  knotty  point  there  is  a disagree- 
ment among  authorities,  and  so  we  may  be  content 
to  leave  it,  since  the  question  is  hardly  one  which  is 
capable  of  a practical  solution. 

A phenomenon  to  which  it  is  scarcely  doubtful 
that  Mendelian  principles  apply  is  that  of  sex.  In 
the  male  and  female  sexes  of  the  majority  of 
animals  we  have  a very  clear  example  of  a pair  of 
definite  differentiating  characters.  And  the  fact  that 
in  the  majority  of  forms  the  two  sexes  make  their 
appearance  in  nearly  equal  numbers,  may  be  thought 
to  point  clearly  to  the  conclusion  that  the  separation 
of  the  sexes  depends  upon  some  quite  simple  gametic 
process.  Light  has  recently  been  thrown  upon  this 
question  from  the  side  of  the  study  of  the  minute 
structure  of  the  gametes,  and  we  shall  defer  the  further 
discussion  of  the  problem  to  the  chapter  which  deals 
with  microscopic  phenomena  within  the  cell. 

A proper  understanding  of  Mendel’s  law  enables  us 
to  escape  certain  theoretical  difficulties  which  have 
long  been  prominent  in  the  minds  of  students  of 
evolution.  Many  evolutionists  were  accustomed  to 
argue  that  a new  form  suddenly  arising  in  the  midst  of 
an  old-established  species  could  not  give  rise  to  a new 
and  permanent  variety  or  elementary  species,  because 
it  would  immediately  be  ‘ swamped ' by  intercrossing 
with  the  parent  species  from  which  it  was  derived. 
If,  however,  the  character  distinguishing  the  new 
type  is  allelomorphic  to  the  corresponding  character, 


222 


MENDELISM 


or  absence  of  a character,  shown  by  the  parent  form 
this  difficulty  disappears.  For  suppose  as  an  extreme 
case  that  the  new  type  arises  as  a single  individual 
only,  which  is  therefore  compelled  to  mate  with  a 
member  of  the  original  species.  If  the  new  character 
is  recessive  it  will  disappear  in  the  immediate  offspring 
of  this  cross.  But  half  the  genn-cells  produced  by 
the  cross-bred  form  will  bear  the  new  character  pure 
and  undiluted.  If  any  of  these  cross-breds  mate  to- 
gether the  new  type  wiU  appear  in  a quarter  of  their 
offspring.  Even  if  all  of  them  mate  with  members  of 
the  original  t5q)e,  half  the  offspring  of  such  matings 
will  be  heterozygous,  and  sooner  or  later  the  hetero- 
zygotes will  be  sure  to  mate  with  one  another,  and 
give  rise  once  more  to  the  novel  type  of  individuals. 
If  the  new  form  has  any  structural  or  other  advantage 
over  the  old  species,  the  former  will  tend  to  survive  at 
the  expense  of  the  parent  type,  and  it  may  survive 
if  it  is  only  equally  w'ell  fitted  for  the  battle  of  life.  In 
the  case  of  dominance  of  the  mew  form  the  same 
process  will  take  place,  only  it  will  be  apparently  more 
rapid  in  the  early  stages  because  the  cross-breds  will 
themselves  exhibit  the  new  character.  In  this  case,  even 
if  the  new  type  has  a very  marked  advantage  over  the 
parent  form,  the  process  of  completely  supplanting  the 
latter  will  be  delayed,  because  the  old  type  of  character 
can  survive  concealed  in  heterozygote  individuals. 

Let  us  pause  for  a moment  to  sum  up  the  novel  ideas 
which  have  so  far  been  presented  in  this  and  the 
preceding  chapter. 

We  found  in  the  first  place  that  irom  the  point 


SUMMARY 


223 


of  view  of  heredity  we  must  look  upon  an  animal  or  a 
plant  as  a composite  being,  made  up  of  a great  number 
of  unit  characters,  each  capable  of  separate  description, 
and  all  inherited  independently  of  one  another. 

When  a pair  of  nearly-related  animals  or  plants 
are  mated  together,  when,  in  fact,  like  is  bred  with 
like,  and  with  still  greater  certainty  in  cases  of  self- 
fertilization  such  as  are  not  uncommon  among  plants, 
every  unit  character  borne  by  one  gamete  finds  a 
corresponding  mate  among  the  characters  borne  by  the 
second  gamete.  It  naturally  follows  that  a series  of 
characters  similar  to  those  of  the  parent  or  parents 
make  their  appearance  in  the  offspring. 

When  a pair  of  individuals  belonging  to  distinct 
varieties  or  races  are  mated  together,  the  result  is  the 
same  in  the  case  of  the  majority  of  characters  exhibited 
by  each  of  them.  For  separate  varieties  of  the  same 
species  differ  from  one  another  in  a small  number  of 
units  only,  and  organisms  which  differ  in  more  than  a 
few  unit  characters  refuse  altogether  to  unite  for  the 
production  of  offspring.  From  the  study  of  the  precise 
behaviour  of  those  characters  in  which  a pair  of 
parental  organisms  differ,  a flood  of  light  has  been 
thrown  upon  the  phenomena  of  inheritance. 

We  find,  as  a rule,  that  opposed  to  every  differen- 
tiating unit  character  of  one  parent  there  exists  a 
corresponding  but  different  character  in  the  other 
parent.  One  parent  may  have  smooth  seeds  and  the 
other  wrinkled  seeds,  for  example.  Very  frequently 
the  corresponding  feature  consists  in  the  absence — or 
failure  to  appear — of  a particular  character,  as,  for 


224 


MENDELISM 


instance,  when  the  non-development  of  pigment  leads 
to  the  appearance  of  white  flowers. 

We  can  now  realize  how  necessary  it  is,  in  order  to 
avoid  hopeless  confusion,  to  follow  the  behaviour  of 
each  pair  of  characters  in  the  offspring  separately. 

The  result  of  the  meeting  between  the  two  opposed 
characters  of  the  same  pair  we  saw  to  be  different  in 
different  cases.  There  may  arise  in  the  offspring 
(i)  the  appearance  of  a simple  blend  of  the  two  parental 
characters.  Or  (2)  one  character  may  be  more  or  less 
dominant  over  the  other.  Or  (3)  the  combination  of 
the  two  parental  characters  in  the  offspring  may  give 
rise  to  an  appearance  quite  different  from  that  of  either 
of  them,  very  much  in  the  same  way  as  in  chemistry 
oxygen  and  hydrogen  when  combined  give  rise  to  water. 
Or  (4)  we  may  get  further  complications  in  which  un- 
suspected characters,  present  in  an  invisible  condition 
in  one  or  both  parents,  take  a part,  often  giving  rise 
to  the  appearance  of  a supposed  reversion. 

The  most  important  phenomenon  of  all,  however, 
is  that  which  is  found  to  occur  at  the  formation  of  the 
germ  cells  of  the  heterozgyote  plant  or  animal.  What- 
ever the  appearance  of  the  hybrid  form  may  have  been, 
at  this  stage  in  its  history  the  determining  factors  for 
each  member  of  the  pair  of  parental  allelomorphs 
reappear  in  their  entirety  in  certain  cells  which  by 
their  division  give  rise  to  the  gametes,  and  at  one 
of  the  divisions  in  question  the  parental  characters  (in 
a potential  condition)  separate  completely  from  one 
another,  so  that  half  the  gametes  bear  one  allelomorph 
and  half  of  them  the  other.  In  cases  where  more  than 


SUMMARY 


225 


one  pair  of  allelomorphs  has  taken  part  in  the  cross,  the 
members  of  each  pair  are  found,  as  a rule,  to  undergo 
this  process  of  segregation  quite  independently  of  all 
the  other  pairs. 

The  result  of  this  phenomenon  of  segregation  is 
that  we  find  our  conception  of  what  constitutes  purity 
in  a strain  of  animals  or  plants  to  be  completely 
altered.  We  now  know  that  purity  does  not  depend 
upon  the  length  of  time  during  which  the  race  has 
exhibited  a constant  character.  A strain  of  absolute 
purity  may  arise  from  the  second  generation  of  a cross. 
Such  a pure  strain  may  show  an  entirely  new  com- 
bination of  the  parental  characters.  But  this  is  so 
far  the  only  kind  of  novelty  which  we  can  produce  at 
will.  We  know  almost  nothing  as  to  the  method  by 
which  entirely  new  characters  arise.  We  can  only 
take  advantage  of  such  characters  when  they  happen 
to  make  their  appearance. 

I would  draw  special  attention  to  the  definiteness  of 
the  characters  with  which  we  deal.  We  do  not  evoke 
improved  features  by  gradual  selection ; the  characters 
are  either  there  or  they  are  not.  Let  it  be  further 
remembered  that  every  process  of  this  kind  which  has 
been  worked  out  in  the  case  of  a plant  can  be  paralleled 
by  similar  phenomena  taking  place  in  some  one  or 
other  of  the  higher  animals. 

On  the  mind  of  a biologist  familiar  with  what 
was  known  of  heredity  only  about  twenty  years 
since,  these  facts  must  fall  with  a sense  of  complete 
novelty.  The  ideas  current  even  so  short  a time  ago 
are  not  so  much  extended,  or  even  altered,  as  replaced 

15 


226 


MENDELISM 


by  an  entirely  new  set  of  ideas.  And  it  may  be 
remarked  in  passing  that  the  biologist  of  fifty  years 
ago  and  more  was  much  nearer  to  our  present  line  of 
inquiry. 

We  have  seen  enough  to  enable  us  to  recognise 
very  clearly  the  vital  importance  of  an  understanding 
of  the  constitution  of  the  gametes  in  all  questions  of 
heredity.  There  must  exist  in  the  gametes,  in  an 
uncombined  condition,  those  units  which  by  their 
combination  in  zygotic  organisms  lead  to  the  appear- 
ance of  the  characters  which  we  can  recognise.  But 
we  have  seen  that,  owing  to  the  appearance  of  . domi- 
nance and  other  kindred  phenomena,  the  visible  external 
characters  of  an  organism  are  not  a complete  guide  to 
the  nature  of  its  gametes.  It  is  only  by  careful 
breeding  that  we  can  distinguish  the  heterozygote  from 
the  pure  dominant  form — ^to  take  the  simplest  possible 
example  of  this  difficulty.  For  this  reason  it  has  now 
become  the  chief  business  of  the  student  of  heredity 
to  determine  by  experiment  what  combinations  of 
allelomorphs  are  present  in  the  gametes  of  the  indi- 
viduals with  which  he  is  working. 

The  behaviour  of  these  allelomorphs  has  now  been 
disentangled  in  many  cases  of  very  considerable  com- 
plexity ; and  all  such  cases  as  have  been  so  far  examined 
in  detail  have  proved  explicable  in  terms  of  a larger  or 
smaller  number  of  allelomorphic  pairs,  all  of  which 
obey  Mendel’s  law — ^with  the  single  exception  of  those 
cases  in  which  coupling  between  the  allelomorphs  of 
different  pairs  introduces  a slight  further  complication. 


MENDELISM  AND  BIOMETRY 


227 


Although  it  is  perhaps  scarcely  probable  that  Mendel’s 
law  will  ultimately  prove  universal  in  its  application, 
nevertheless  the  few  exceptions  recorded  by  competent 
observers  still  require  further  examination  before  they 
can  be  accepted  as  invalidating  the  law  in  any  single 
instance. 

The  question  naturally  arises  as  to  how  far  the  Men- 
delian  rule  of  inheritance  agrees  with  or  contradicts 
those  estimations  of  hereditary  values  which  have  been 
arrived  at  by  the  labours  of  the  biometricians. 

So  long  ago  as  1902  Mr.  G.  Udny  Yule  endeavoured 
with  some  apparent  success  to  reconcile  the  Mendelian 
results  with  those  of  biometry.  Progress  has  been 
rapid  during  the  subsequent  years,  and  what  we  have 
now  before  us  is  rather  the  question  of  reconciling  the 
biometrical  conclusions  with  the  firmly  established 
facts  of  Mendelian  inheritance. 

In  1902  Yule  considered  the  case  of  a pair  of  simple 
Mendelian  characters,  A and  a,  exhibited  in  a mixed 
population  breeding  together  at  random,  in  such  a way 
that  the  total  number  of  germ  cells  bearing  A and  a 
respectively  might  be  regarded  as  equal  in  any  genera- 
tion. In  such  a case  it  will  always  be  an  even  chance 
whether  a recessive  parent  will  produce  a dominant 
or  a recessive  child,  because  the  chance  of  its  gamete 
(a)  mating  with  A or  « is  the  same.  A knowledge 
of  the  ancestry  of  the  recessive  parent  makes  no 
difference  to  the  result.  Consequently  the  case  of 
the  pure  recessive  does  not  fall  in  with  any  possible 
theory  of  ancestral  heredity. 


15—2 


228 


MENDELISM 


But  on  turning  to  the  dominant  parent,  the  case  is 
found  to  be  different.  For  such  an  one  may  be  either 
a pure  dominant  homozygote  giving  off  ^-gametes 
only,  or  it  may  be  a heterozygote  giving  off  equal 
numbers  of  A-  and  a-gametes.  Yule  shows  that  if  both 
the  parents  of  the  A individual  exhibited  the  character 
A , the  proportionate  number  of  its  offspring  which  may 
on  the  average  be  expected  to  show  the  A character  is 
greater  than  would  have  been  the  case  if  one  of  its 
parents  exhibited  the  character  a.  And  in  a similar 
way  a knowledge  of  the  characters  shown  by  the  grand- 
parents adds  something  to  the  certainty  of  the  pre- 
diction as  to  the  proportionate  numbers  of  offspring  of 
the  two  kinds  which  are  to  be  expected,  when  the 
average  of  a number  of  cases  is  taken  according  to  the 
usual  statistical  method. 

Yule  therefore  regarded  the  case  of  the  dominant 
character  as  showing  conformity  with  the  law  of 
ancestral  heredity,  according  to  his  own  statement  of 
that  generalization,  which  was  to  the  following  effect : 
The  law  that  ‘ the  mean  character  of  the  offspring  can  he 
calculated  with  the  more  exactness,  the  more  extensive 
our  knowledge  of  the  corresponding  characters  of  the 
ancestry,  may  be  termed  the  law  of  ancestral  heredity.’* 

It  may  be  remarked  in  passing  that  Yule’s  dis- 
tinction of  the  problems  of  genetics  into  those  of 
intra-racial  heredity  and  those  of  hybridization  cannot 
now  be  regarded  as  holding  good,  unless  the  term 
hybridization  is  to  be  extended  to  many  cases — e.g., 
tjiat  of  the  inheritance  of  coat  colour  in  thoroughbred 
♦ ‘ New  Phytologist,*  vol.  i.,  p.  202. 


MENDELISM  AND  BIOMETRY 


229 


horses,  which  would  have  been  classed  unhesitatingly 
as  instances  of  heredity  by  all  biometricians  in  1902. 
Bateson’s  instinct  did  not  fail  him  when  he  divided 
these  problems  into  those  of  continuity  and  those  of 
discontinuity  respectively,  although  at  the  present  time 
the  realm  of  continuous  variation  and  inheritance  is 
being  steadily  encroached  upon  owing  to  the  analysis 
of  complex  characters  into  definite  constituents. 

In  1904  Karl  Pearson  struck  a blow  at  the  prospect 
of  conformity  between  biometrical  and  Mendelian 
results  in  his  memoir,  ‘ On  a Generalized  Theory  of 
Alternative  Inheritance,  with  special  reference  to 
Mendel’s  Laws.’  Pearson’s  treatment  of  the  subject 
involved  advanced  mathematical  reasoning,  and  we 
can,  therefore,  only  give  a brief  summary  of  his  main 
results.  Pearson  proposes  special  terms  for  the  A and 
the  a elements  respectively  of  a couplet  or  pair  of 
allelomorphs.  He  proposes  to  call  the  A element  a 
frotogene,  and  the  a element  an  allogene,  and  he  thus 
distinguishes  between  the  two  sorts  of  homozygotes 
by  calling  AA  a protozygote  and  aa  an  allozygote. 

Pearson  considered  the  case  of  a population  breeding 
together  at  random,  in  which  a single  measurable 
character,  such  as  stature,  is  determined  by  the  combined 
action  of  an  indefinite  numiber  of  pairs  of  allelomorphs, 
and  he  proceeded  to  work  out  the  value  of  parental 
correlation  which  was  to  be  expected  under  these 
circumstances.  This  value  he  found  to  be  exactly 
one-third,  a value  which  happens  to  be  identical  with 
Galton’s  original  determination  of  parental  correlation 
from  his  statistics  of  human  stature.  A considerable 


230 


MENDELISM 


number  of  determinations  of  parental  correlation 
have,  however,  since  been  made  in  the  case  of  all 
kinds  of  characters.  The  values  show  considerable 
variation,  but  the  average  which  they  indicate  is  much 
nearer  to  0’5  than  to  0-33.  Pearson  therefore  con- 
cluded that  in  none  of  these  cases  could  anything 
resembling  Mendelian  inheritance  be  taking  place, 
and  that  the  latter  is,  in  fact,  the  exception  rather  than 
the  rule.* 

Mendelians,  aware  of  the  certainty  of  their  own 
results,  and  being  convinced  that  these  facts  must 
have  a very  wide  application,  were  thereupon  driven 
reluctantly  to  the  conclusion  that  something  was 
seriously  wrong  with  the  methods  adopted  by  biome- 
tricians for  determining  the  coefficients  of  correlation. 
It  seems,  however,  that  this  conclusion  may  have  been 
arrived  at  with  undue  haste. 

In  August,  1906,  Mr.  Yule  read  before  the  Inter- 
national Congress  of  Hybridization  assembled  in 
London  a very  interesting  paper  on  ‘ The  Theory  of 
Inheritance  of  Quantitative  Compound  Characters  on 
the  basis  of  Mendel’s  Laws.’  Though  some  difficulty 
was  then  experienced  in  following  his  argument  by 
an  audience  unaccustomed  to  statistical  methods, 
Yule’s  conclusion  is  really  very  simple. 

Yule  points  out  that  the  only  character  dealt  with 
in  Pearson’s  memoir  is  the  number  of  protogenicor 
allogenic  couplets  present  in  the  individual,  and  it  is  the 
proportionate  number  of  these  couplets  present  in  the 
parent  and  in  the  offspring  respectively  which  is  taken 
as  determining  the  value  of  the  correlation  coefficient. 


MENDELISM  AND  BIOMETRY 


231 


Consequently  Pearson’s  treatment  of  the  subject  does 
not  justify  his  statement  that  the  Mendelian  theory 
gives  a rigid  value  for  the  coefficients  of  parental  cor- 
relation for  all  races  and  characters — a conclusion 
which  he  regards  as  fatal  to  this  theory,  because  the 
coefficients  for  different  characters  and  races,  as  found 
statistically,  show  considerable  individual  differences, 
and  seem  to  cluster  round  a value  considerably  higher 
than  that  indicated  by  his  elaboration  of  the  theory  of 
the  pure  gamete.  Yule  thereupon  discusses  a somewhat 
more  general  case,  and  considers  the  inheritance  of  a 
length  made  up  of  a number  of  distinct  segments,  each 
of  which  is  determined  by  an  independent  pair  of 
allelomorphs.  Supposing  each  segment  to  take  the 
length  a,  b,  or  c,  according  as  the  corresponding  proto- 
zygote, heterozygote,  or  allozygote  is  present.  Yule 
arrives  at  an  equation  from  which  the  correlation 
between  parent  and  offspring  may  be  found.  From 
that  equation  the  following  results  are  deducible  : 

If  there  is  dominance — i.e.,  ii  a = b,  or  b-c,  the  corre- 
lation coefficient  is  the  same  as  that  found  by  Pearson 
— i.e.,  one-third. 

But  if  the  heterozygote  always  gives  rise  to  a 
length  exactly  intermediate  between  those  due  to 
the  respective  homozygotes,  the  correlation  is  found 
to  be  one-half. 

Cases  of  partial  dominance  will  give  an  intermediate 
value.  Consequently,  according  to  the  degree  of 
imperfection  of  dominance,  and  without  assuming  any 
other  disturbing  circumstances,  values  of  parental 
correlation  varying  from  0‘33  to  0-5  are  to  be  expected 


232 


MENDELISM 


on  the  Mendelian  theory  of  inheritance  when  applied 
to  populations.  These  figures  are  calculated  on  the 
supposition  that  there  is  random  mating  of  the  parents, 
but  if  there  were  a tendency  for  like  to  mate  with  like 
the  correlation  values  would  become  still  higher.  Yule 
therefore  concludes  that  ‘ there  is  therefore  no  diffi- 
culty in  accounting  for  a coefficient  of  0-5  on  the 
theory  of  segregation,  but  such  a value  probably 
indicates  an  absence  of  the  somatic  phenomenon  of 
dominance.  In  the  case  of  characters  like  stature, 
span,  etc.,  in  man  this  does  not  seem  very  improb- 
able.’ 

It  is  impossible  to  bring  the  present  chapter  to  a con- 
clusion without  some  reference  to  the  practical  aspects 
of  the  Mendelian  discovery.  The  progress  of  experi- 
mental research  in  this  field  during  the  last  few 
years  has  been  so  rapid,  that  there  is  little  ground  for 
astonishment  in  the  fact  that  only  a small  proportion 
of  those  to  whom  the  discovery  of  the  Mendelian 
method  is  of  the  very  highest  importance  from  a com- 
mercial point  of  view  have  yet  arrived  at  any  serious 
appreciation  of  it.  The  improvement  of  the  breeds  of 
cultivated  plants  and  domestic  animals  is  a subje«t  of 
vital  importance  to  the  whole  human  race,  quite  apart 
from  the  question  of  the  commercial  profit  which  it 
represents  for  those  whose  business  it  is  to  be  directly 
concerned  with  the  process — ^the  actual  plant-  and 
animal-breeders  themselves. 

Hitherto  the  methods  of  amelioration  which  have 
been  adopted  have  depended  largely  upon  guess-work. 


PRACTICAL  APPLICATIONS 


233 


or  at  the  best  upon  the  result  of  practical  experience. 
We  are  now  within  sight  of  the  day  when  a complete 
system  of  precise  scientific  methods  will  have  been 
elaborated.  The  time  required  for  the  development 
and  application  of  these  methods  must  chiefly  depend 
upon  the  apathy  or  enterprise  of  those  in  whose  hands 
rests  the  means  of  subsidizing  this  kind  of  work,  for 
without  proper  resources  the  progress  of  any  such 
study  must  of  necessity  be  slower  than  it  would  be 
if  properly  - equipped  establishments  were  at  the 
disposal  of  duly  trained  experimenters  receiving  an 
adequate  remuneration. 

The  practical  application  of  Mendelism  cannot  be 
better  illustrated  than  by  an  account  of  Prof.  R.  H. 
Biffen’s  work  upon  the  improvement  of  cereals,  particu- 
larly of  wheat — work  which  exhibits  an  extraordinary 
contrast  in  point  of  scientific  exactness  with  everything 
of  the  kind  which  has  been  previously  undertaken. 
This  contrast  was  remarkably  displayed  at  one  of  the 
morning  sessions  of  the  recent  International  Congress 
on  Hybridization  and  Plant  Breeding,  held  under  the 
auspices  of  the  Royal  Horticultural  Society.  On  that 
occasion  a series  of  communications  upon  the  subject 
of  cereals  culminated  in  an  admirable  account  given  by 
I'rof.  Biffen  of  the  way  in  which  the  problems  of  their 
improvement  have  been  overcome  at  the  experimental 
farm  of  the  Cambridge  University  Department  of 
Agriculture.  And  it  was  a gratifying  sign  of  better 
times  to  observe  the  enthusiastic  interest  with  which 
practical  men  greeted  his  communication. 

As  a preliminary  measure  Biffen  has  worked  out  the 


234 


MENDELISM 


inheritance  of  a number  of  comparatively  simple 
characters,  many  of  which  have  little  practical  impor- 
tance. But  the  fact  of  their  strictly  Mendelian  be- 
haviour showed  the  possibility  of  readily  obtaining  any 
desired  combination  of  them,  and  at  the  same  time 
rendered  it  highly  probable  that  characters  of  a more 
practical  value  to  the  farmer  would  prove  similarly 
amenable  to  the  breeder’s  art. 

Thus  Biffen  found  that  the  following  pairs  of 
characters,  among  others,  exhibited  simple  Mendelian 
phenomena,  the  one  placed  first  being  in  each  case  the 
dominant : 


Beardless  ears. 

Keeled  glumes. 

Felted  glumes. 

Red  chaff. 

Red  grain. 

Thick  and  hollow  stem, 


Bearded  ears; 

Round  glumes. 
Glabrous  glumes^ 
White  chaff. 

White  grain. 

Thin  and  solid  stem. 


And  so  on.  In  other  cases,  again,  the  Fj  generation 
showed  a character  intermediate  between  those  of 
the  parents,  and  in  Fg  there  appeared  a ratio  corre- 
sponding to  A : 2Aa  : a. 

Thus  when  Polish  wheat  (early)  was  crossed  with 
Rivet  wheat  (late),  the  time  of  ripening  of  the  Fj 
generation  was  intermediate  between  those  of  the 
parents.  In  Fj,  103  early,  210  intermediate,  and 
100  late  plants,  were  counted.  Time  of  ripening  is, 
moreover,  clearly  a character  which  may  be  of  con- 
siderable practical  importance. 

In  further  illustration  of  what  can  be  done  from  a com- 
mercial point  of  view,  we  will  consider  the  case  of  two 
other  characters  only — rust  immunity  and  ‘ strength.’ 


PRACTICAL  APPLICATIONS 


235 


There  is  a quality  of  wheat  grains  known  as  strength 
which  is  essential  for  the  production  of  a flour  such  as 
can  be  baked  into  the  kind  of  loaf  which  is  at  present 
the  only  one  saleable  in  England.  This  quality  un- 
fortunately happens  to  be  wanting  in  all  the  strains  of 
wheat  which  it  has  hitherto  been  possible  to  grow  at 
a profit  in  this  country.  For  this  reason  imported 
American  and  Canadian  hard  wheats,  which  possess 
this  quality  of  strength,  are  worth  in  England  some 
shillings  a quarter  more  than  home-grown  wheats. 

When  such  strong  American  varieties  are  grown  in 
this  country  the  majority  of  them  are  rapidly  found  to 
lose  this  quality,  and  to  become  after  a short  time  as 
‘ weak  ’ as  ordinary  English  wheats.  Some  of  them  do, 
however,  retain  their  strength,  and  after  several  seasons 
— in  one  case  fourteen — show  no  signs  of  deterioration. 
An  example  of  a wheat  of  this  latter  type  is  afforded 
by  Red  Fife,  which  is  the  basis  of  the  mixed  wheat 
known  commercially  as  Manitoba  Hard,  the  latter 
consisting,  as  a matter  of  fact,  of  a mixture  of  several 
different  varieties.  Unfortunately  these  permanently 
hard  wheats  do  not  yield  so  large  a crop  as  the  com- 
monly cultivated  English  varieties,  and  so  their  higher 
price  does  not  make  up  for  the  smaller  number  of 
bushels  per  acre  obtained  when  they  are  grown. 

Biffen  therefore  set  to  work  upon  the  problem  of 
combining  hardness  or  strength  with  the  power  of 
yielding  a good  crop,  and  with  the  other  desirable 
qualities  characteristic  of  the  home-grown  varieties. 
With  this  end  in  view  Manitoba  Hard  was  crossed  with 
a typical  English  wheat — Rough  Chaff. 


236 


MENDELISM 


The  Fj  plants  produced  grains  all  of  which  were 
fully  as  hard  as  those  of  the  Manitoba  variety. 

These  grains  were  sown,  and  it  was  found  that  some  of 
the  resulting  plants  produced  strong  grains  and  others 
weak  ones,  and  that  the  former  were  to  the  latter  very 
nearly  in  the  numerical  ratio  of  3:1.  Actually  they 
were  as  152  : 48  in  a sample  of  200  taken  at  random. 

In  order  to  obtain  confirmation  of  this  most  impor- 
tant result,  Biffen  sent  samples  of  the  grains  borne 
by  the  plants  to  a well-known  authority  on  milling 
wheats,  requesting  his  judgment  upon  them,  but  with- 
out telling  him  their  manner  of  origin.  The  answer 
was  even  more  satisfactory  than  could  possibly  have 
been  anticipated.  Certain  of  the  samples  were  stated 
by  the  expert  to  belong  to  the  variety  Red  Fife, 
which  is  the  name  of  the  particular  strain  of  Manitoba 
Hard  originally  made  use  of  in  the  experiments, 
whilst  others  were  assigned  to  a definite  strain  of 
ordinary  weak  English  wheat.  The  segregation  of 
these  characters  was,  therefore,  complete,  strength 
being  a Mendelian  dominant  to  weakness. 

In  the  next  generation  certain  of  the  dominant 
plants,  as  was  to  be  expected,  bred  true,  and  amongst 
them  were  individuals  which  combined  with  strength 
of  grain  the  other  desirable  qualities  of  the  second 
parent.  The  problem  has,  therefore,  been  completely 
solved,  and  there  can  be  little  doubt  that  whe.n  these 
new  types  are  brought  into  general  cultivation  the 
profit  obtainable  from  the  growing  of  wheat  in  this 
country  will  be  increased  by  several  shillings  to  the 
acre  of  crop  grown. 


PRACTICAL  APPLICATIONS 


237 


We  may  next  turn  to  an  even  more  important 
achievement.  In  many  countries  the  annual  loss  of 
crop  due  to  the  attacks  of  yellow  rust,  Puccinia 
glumarum,  amounts  on  a moderate  estimate  to  a con- 
siderable number  of  millions  of  pounds  sterling. 
Certain  strains  of  wheat  exist,  indeed,  which  are  more 
or  less  completely  immune  to  the  ravages  of  this  fungus, 
but  these  are  usually  wanting  in  other  qualities  which 
are  indispensable  to  the  farmer.  If  it  should  be  found 
that  immunity  to  rust  is  a simple  Mendelian  allelo- 
morph, it  would  be  possible  to  combine  this  quality 
with  any  other  useful  character  which  obeyed  the  same 
law  of  inheritance — as  several  useful  characters  have 
already  been  shown  to  do.  At  one  time  it  must  have 
been  thought  that  a similar  method  of  inheritance  of 
the  character  rust -immunity  was  too  excellent  a boon 
to  be  reasonably  hoped  for. 

Among  a great  number  of  strains  of  wheat  grown 
on  the  Cambridge  experimental  farm,  several  types 
showed  marked  differences  in  the  degree  of  their 
immunity  from,  or  susceptibility  to,  the  attacks  of 
Puccinia  glumarum.  Among  them  Mr.  Biffen  found 
one  which  was  apparently  quite  immune,  and,  though 
grown  in  the  midst  of  numbers  of  rusted  plants,  itself 
never  showed  a trace  of  infection.  Of  another  type, 
known  as  Michigan  bronze,  no  single  individual  ever 
escaped  the  rust,  and  so  badly  were  the  plants  of  this 
strain  diseased  that  very  few  ripe  grains  could  ever  be 
obtained  from  them. 

Biffen  crossed  these  two  types  together.  In  the 
first  generation  every  plant  without  exception  was 


238 


MENDELISM 


badly  rusted,  but  fortunately  a considerable  number 
of  ripe  grains  was  obtained,  and  these  were  sown 
to  produce  the  second  generation.  When  the  plants 
of  this  generation  had  grown  up  it  was  observed 
that  among  a majority  of  badly-rusted  plants  certain 
individuals  stood  out  fresh  and  green,  being  entirely 
free  from  infection.  On  examination  it  was  found  that 
every  plant  could  be  placed  in  one  or  other  of  two 
categories — either  it  was  badly  rusted  or  it  was 
entirely  free  from  rust ; and  the  numbers  of  the  two 
kinds  of  plants  were  as  follows  : 1,609  infected,  523 
immune. 

It  is  clear,  then,  that  immunity  and  susceptibility 
to  the  attacks  of  yellow  rust  behave  as  a simple  pair  of 
Mendelian  characters,  immunity  being -recessive.  And 
it  is,  therefore,  possible  to  obtain  by  crossing,  in  three 
generations,  a pure  rust-free  strain  containing  any  other 
desired  quality  which  is  similarly  capable  of  definite 
inheritance. 


BIBLIOGRAPHY. 

Mendel,  G.  J.  : Briefe  an  Carl  Nageli. 

Bateson,  W.  : Mendel’s  Principles  of  Heredity,  1909. 
Bateson,  W.  ; Punnett,  R.  C.  ; and  Saunders,  Miss  E.  R.  : 
Reports  to  the  Evolution  Committee  of  the  Royal  Society, 
1902-1909. 

The  Journal  of  Genetics,  1910 — 

Punnett,  R.  C,  ; Mendelism,  fifth  edition,  1919. 


CHAPTER  IX 


RECENT  CYTOLOGY 

Every  living  creature  may  be  regarded  as  being  built 
up  of  a number  of  structural  units  which  are  known 
as  cells.  In  the  case  of  some  of  the  simplest  animals 
and  plants,  indeed,  the  whole  body  of  the  organism  is 
composed  of  a single  cell — a small  mass  of  living  proto- 
plasm, containing,  as  a rule,  only  one  nucleus.  But 
in  all  the  higher  animals  and  plants  the  adult  body  is 
made  up  of  a great  number  of  such  cells  living  in 
intimate  association  with  one  another. 

The  living  material  of  which  the  cell  is  composed  is 
known  as  protoplasm.  Protoplasm  is  a highly  com- 
plicated and  unstable  combination  of  substances, 
amongst  the  constituents  of  which  the  chemical 
elements,  carbon,  oxygen,  hydrogen,  nitrogen,  and 
sulphur,  play  the  chief  parts.  Its  consistency  is  slimy 
and  semifluid. 

Concerning  the  nucleus — ^the  most  essential  and 
characteristic  of  ceU  organs — more  will  have  to  be  said 
later  on.  Other  important  organs  of  cells  are  a wall 
or  membrane  which  externally  surrounds  them,  one 
or  more  vacuoles  or  cavities  containing  a watery  fluid, 
or  sometimes  a gas,  and  a certain  number  of  more  solid 


240 


RECENT  CYTOLOGY 


bodies  or  flastids.  Certain  plastids  present  in  the 
majority  of  plants  are  of  particular  importance  as  con- 
taining the  green  substance  chlorophyll,  which  plays 
an  essential  part  in  the  fixation  of  carbon  from  the 
atmosphere. 

Amongst  unicellular  organisms  — the  creatures 
already  mentioned  as  being  made  up  of  a single  cell 
only — those  which  contain  chlorophyll  and  are  pro- 
vided with  a firm  cell  wall,  built  up  of  a material 
known  as  cellulose,  are  usually  regarded  as  simple 
plants ; whilst  those  in  which  chlorophyU  and  a cell 
wall  are  absent  aie  looked  upon  as  simple  types  of 
animals.  Similarly  slight  differences  distinguish  the 
cells  which  build  up  the  fabric  of  the  higher  plants 
from  those  of  which  the  bodies  of  the  more  complicated 
animals  are  composed,  so  that  in  almost  all  essential 
points  an  account  of  the  behaviour  of  the  cells  of  the 
members  of  one  kingdom  will  apply  equally  well  to 
those  of  the  other.  After  a few  further  preliminary 
remarks  we  shall,  therefore,  for  the  sake  of  simplicity, 
speak  of  a generalized  type  of  cell,  the  behaviour  of 
which,  except  in  points  of  detail,  will  resemble  that  of 
the  actual  cells  of  plants  or  animals  indifferently.  But 
in  order  to  convey  a more  definite  idea  of  an  uni- 
cellular animal  to  those  who  are  unfamiliar  with  the 
rudiments  of  Elementary  Biology,  we  may  refer  briefly 
to  the  well-known  form  Amceba,  which  will  serve  as 
an  excellent  type  of  an  animal  consisting  of  a single 
free-living  cell. 

This  little  creature  consists  of  a mass  of  protoplasm 
enclosing  a nucleus  which  is  more  or  less  centrally 


THE  CELL 


241 


situated  and  approximately  spherical  in  form.  The 
protoplasm  is  divided  into  an  outer  hyaline  and  an 
inner  granular  portion,  the  former  being  limited  exter- 
nally by  a very  delicate  membrane.  The  shape  of  the 
animal  is  irregular,  and,  moreover,  undergoes  gradual 
alteration  owing  to  the  characteristic  amoeboid  move- 
ments. These  consist  in  a slow  protrusion  and  with- 
drawal of  processes  of  the  body,  enabling  the  animal 
to  change  its  position  by  a kind  of  flowing  movement, 
and  also  to  engulf  its  food,  which  consists  of  various 


Fig.  17.— Amoeba. 

n,  Nucleus ; /,  food  particle. 


Fig.  18.— Pleurococcus. 

n,  Nucleus ; w,  cell -wall ; 
chi,  chloroplast. 


minute  organic  particles,  by  the  simple  process  of 
flowing  around  it. 

In  contrast  with  Amceha  the  unicellular  plant  Pleuro- 
coccus  is  motionless,  and  is  surrounded  by  a firm  wall 
of  cellulose.  In  addition  to  a central  nucleus,  the 
plant  contains,  embedded  in  its  peripheral  protoplasm, 
several  plastids  which  bear  the  chlorophyll  concerned 
in  the  assimilation  of  carbon  from  the  gases  of  the 
atmosphere.  This  chlorophyll  lends  a green  colour  to 
the  whole  contents  of  the  cell,  and  in  its  natural  habitat 
the  plant  is  quite  conspicuous.  The  green  powdery 
substance  often  to  be  seen  on  the  bark  of  trees, 


243 


RECENT  CYTOLOGY 


especially  on  the  side  turned  to  the  north,  and  in 
similar  shady  situations,  consists,  as  a rule,  of  great 
numbers  of  minute  Pleurococcus  plants,  although  the 
size  of  a single  specimen  may  be  represented  by  a 
diameter  of  little  more  than  the  two-thousandth  part 
of  an  inch. 

We  are  more  particularly  concerned,  however,  with 
the  higher  animals  and  plants,  the  bodies  of  which  are 
built  up  of  a great  number  of  separate  cells.  Some 
of  these  cells  may  be  modified  in  various  ways,  but  they 
all  conform,  at  least  in  the  youthful  condition,  to  types 
not  far  removed  from  those  of  Aniceba  and  Pleurococcus 
respectively.  Certain  parts  of  these  higher  organisms, 
indeed,  such  as  the  bones  of  vertebrate  animals  and 
the  wood  of  trees,  do  not  consist  solely  of  living  cells, 
but  are  composed  to  a great  extent  of  dead  material 
excreted  or  built  up  by  the  activity  of  living  cells. 
These  latter  have,  then,  either  ceased  to  live,  or  they 
may  continue  to  exist  in  the  interstices  of  the  hard 
skeletal  framework. 

New  cells  come  into  existence  in  only  one  way — 
namely,  by  a process  of  division  which  takes  place  in  a 
pre-existing  cell.  In  comparatively  rare  cases  a cell 
may  give  off  a small  bud  which  forthwith  develops 
into  a new  cell  like  the  old  one.  In  such  a case  we  may 
speak  of  the  cell  which  gives  off  the  bud  as  the  mother- 
cell, and  of  the  cell  into  which  the  bud  develops  as 
, the  daughter-cell.  But  by  far  the  most  frequent 
; method  of  cell-reproduction,  and  the  only  one  which 
I is  characteristic  of  the  higher  animals  and  plants,  takes 
I place  by  the  equal  division  of  an  old  cell  into  two 


THE  CELL 


243 


new  ones.  In  this  case,  it  is  only  by  a stretch  of 
language  that  we  can  speak  of  parent-  and  daughter- 
cells,  for  the  individuality  of  the  pre-existing  cell  is 
completely  lost,  and  two  fresh  individualities  have  now 
taken  its  place. 

Since  all  the  cells  of  the  animal  or  plant  body  arise 
by  the  bipartition  of  pre-existing  cells,  it  is  clear  that  if 
we  follow  these  processes  far  enough  back,  in  the  case 
of  any  individual  organism,  we  may  arrive  at  a period 
at  which  only  one  cell  was  present.  And  under 
ordinary  circumstances  this  is  actually  the  case. 
Every  individual  among  the  higher  animals  and  plants, 
iarising  by  the  ordinary  sexual  method,  existed  at  the 
earliest  stage  of  its  embryonic  history  in  the  form  of  a 
single  cell,  the  fertilized  ovum.  And  the  first  obvious 
process  in  the  development  or  embryology  of  the  young 
organism  consisted  in  the  division  of  this  primitive 
cell  into  two  new  cells.  Each  of  these  new  cells  then 
divided  again  in  like  manner,  and  the  multiplication  of 
cells  continued  until  all  the  innumerable  cells  which 
build  up  the  organs  of  the  adult  body  had  finally 
come  into  existence.  When  growth  is  completed  ceU- 
divisions  continue  more  slowly,  producing  new  cells  to 
make  good  the  wear  and  tear  of  the  bodily  tissues. 

As  the  number  of  cells  increased,  their  relation  to 
one  another  in  space  was  constantly  changing.  Dif- 
ferent cells,  too,  became  modified  in  different  ways ; 
for  instance,  the  cells  on  the  outside  of  the  young 
embryo  took  on  a different  form  from  those  within,  in 
accordance  with  the  different  conditions  to  which 
they  were  exposed,  and  a host  of  other  changes  took 

16—2 


244 


RECENT  CYTOLOGY 


place  too  numerous  for  us  to  follow  in  detail.  Thus 
the  complicated  structure  of  the  adult  organism  was 
gradually  arrived  at  by  a process  of  development  in 
which  cell- multiplication  played  a most  prominent  and 
essential  part. 

We  have  next  to  inquire  what  is  the  method  of  origin 
of  the  original  embryonic  cell — ^the  fertilized  ovum — 
from  which  the  new  animal  or  plant  develops. 

As  is  indeed  implied  by  the  expression  ‘ fertilized 
ovum,’  this  cell  arises  by  the  fusion  together  of  two 
independent  cells,  such  fusion  constituting  the  process 
of  fertilization  or  impregnation.  One  of  the  cells 
which  took  part  in  the  fusion  was  derived  from  one 
parent  organism,  and  bore  the  distinguishing  character- 
istics of  the  cells  which  composed  that  parent — or  at 
least  some  part  of  those  characteristics — ^whilst  the 
other  was  in  like  manner  derived  from  the  second 
parent. 

It  is  to  be  observed  that  this  fusion  together  of  a 
pair  of  cells,  derived  (in  the  case  we  are  considering — 
namely,  that  of  ordinary  biparental  reproduction) 
from  two  separate  individuals,  results  in  the  formation 
of  a complete  new  individuality,  which  arises  definitely 
at  that  point  of  time  at  which  the  fusion  of  the  two 
conjugating  cells  takes  place.  In  this  way  the  cells 
of  the  offspring  are  seen  to  be  of  double  origin,  and  it 
is  found  that  traits  and  characters  derived  from  both 
the  father  and  the  mother  can  co-exist  in  them  side  by 
side. 

The  cells  which  take  part  in  the  above-mentioned 
fusion  are  known  as  gametes,  or  germ-cells — male  and 


THE  CELL 


245 


female  respectively,  according  to  the  sex  of  the  parent 
from  which  each  is  derived.  In  animals  the  female 
gamete  is  known  as  the  ovum,  and  the  male  as  the 
spermatozoon,  and  the  product  of  their  fusion,  as  already 
said,  is  called  the  fertilized  ovum.  Germ-cells  of  a 
similar  kind-  arise  in  a slightly  different  way  in  plants. 
The  germ-cells  are  produced  in  special  parts  of  the 
organism  known  as  the  generative  organs,  which  in 
flowering  plants  are  represented  by  the  pistils  and 
stamens. 

A more  convenient  expression  for  the  fertilized  ovum 
is  that  of  zygote,  a term  which  we  have  previously 
encountered  in  the  shape  of  the  homo-  and  hetero- 
zygotes of  the  Mendelian.  By  an  expansion  of  meaning 
the  term  zygote  is  -also  used  to  express  the  whole 
organism  which  ultimately  arises  from  the  product  of 
fusion  of  a pair  of  gametes,  and  by  this  use  the  impor- 
tance of  the  gamete,  as  opposed  to  the  zygotic  organism 
as  a whole,  is  brought  into  due  prominence. 

We  find,  then,  that  the  succession  of  generations  in 
the  higher  animals  and  plants,  according  to  the  common 
use  of  this  expression,  depends  upon  the  succession 
of  a much  larger  number  of  cell -generations.  By 
repeated  divisions,  each  giving  rise  to  a new  generation 
of  cells,  the  fertilized  ovum  gradually  develops  into 
the  adult  organism.  By  the  division  of  certain 
members  of  the  later  generations  of  cells  which  compose 
this  organism  the  gametes  are  produced.  By  the 
conjugation  of  a pair  of  gametes  a zygote  of  the 
second  generation  arises,  and  the  same  processes  are 
continually  repeated. 


246 


RECENT  CYTOLOGY 


Each  of  the  cells  hitherto  referred  to  possesses  a single 
nucleus,  which  is  usually  a more  or  less  spherical  body 
occupying  a central  position  within  the  cell.  Nuclei, 
like  the  cells  which  contain  them,  arise  only  by  the 
division  of  pre-existing  nuclei.  Thus  the  history  of 
the  nuclei  is  in  every  way  similar  to  the  history  of  the 
cells,  of  which  they  constitute  so  important  a part. 
In  fertilization  the  nuclei  of  the  conjugating  cells  or 
gametes  fuse  together  to  form  the  single  nucleus  of 
the  fertilized  ovum,  and  every  division  of  this  cell,  as 
well  as  of  its  cell-progeny,  is  preceded  by  a division 
of  the  nucleus  into  two  similar  portions. 

We  may  forthwith  concentrate  our  attention  upon 
the  nucleus  as  being  that  part  of  the  cell  which  is  of 
primary  importance  from  the  point  of  view  of  heredity, 
for  it  is  now  generally  recognised  that  the  nucleus 
is  the  part  of  the  cell  in  which  hereditary  features 
are  in  some  way  carried.  And  we  may  next  consider 
a little  more  closely  the  structure  of  the  nucleus  as 
seen  under  high  powers  of  the  microscope. 

In  what  is  somewhat  improperly  called  its  resting 
condition — a condition  which  is  characteristic  of  nuclei 
at  all  times  when  they  are  not  actually  undergoing 
division,  or  preparing  for  that  process — ^the  nucleus  may 
be  seen  to  be  bounded  by  a more  or  less  definite  nuclear 
membrane.  The  internal  structure  of  such  a nucleus  is 
described  as  reticular — that  is  to  say,  at  least  two 
different  substances  are  differentiated  within  the 
nucleus,  one  of  them  forming  a reticulated  meshwork, 
the  interspaces  of  which  are  occupied  by  the  other 
(Fig.  19,  p.  249). 


THE  NUCLEUS 


247 


In  entering  into  a detailed  description  of  the  changes 
which  take  place  in  the  finer  structure  of  the  nucleus, 
it  must  be  clearly  understood  that  the  more  minute 
features  alluded  to  are  only  to  be  seen  with  any  degree 
of  definiteness  in  dead  cells  which  have  been  killed 
practically  instantaneously  by  the  action  of  some 
powerful  chemical  poison.  Under  suitable  conditions 
it  is  believed  that  treatment  of  this  kind  fixes  the  con- 
stituent parts  of  the  nucleus  in  very  nearly  the  same 
relative  positions  as  they  occupied  in  life  at  the 
moment  immediately  preceding  the  death  of  the  cell. 
The  tissues  containing  the  cells  to  be  examined  are  then 
usually  cut  into  very  thin  sections,  and  other  chemicals 
are  applied  to  them,  the  result  of  this  treatment  being 
to  stain  different  parts  of  the  nucleus  of  different 
colours  and  with  different  degrees  of  intensity.  It  is  to 
the  behaviour  of  the  structures  thus  made  visible  that 
our  description  applies,  since  it  is  impossible  to  follow 
these  changes  in  actually  living  cells  except  to  a very 
imperfect  extent.  It  may  be  pointed  out,  however, 
that  we  have  every  reason  for  believing  that  the 
differential  effect  produced  by  the  processes  of  fixing 
and  staining  only  serves  to  render  more  clearly  visible 
real  differences  which  actually  existed  during  the  life 
of  the  cell,  and  some  indications  of  many  of  these 
differences  have  even  been  actually  seen  in  living  cells 
under  exceptionally  favourable  conditions. 

The  nucleus,  when  treated  in  the  manner  described, 
is  seen  to  be  built  up  of  a network  of  branching  fibrils, 
the  meshes  of  which  enclose  a comparatively  clear  and 
hyaline  substance.  The  fibrils  of  the  network  are  made 


248 


RECENT  CYTOLOGY 


up  of  a material  of  comparatively  weak  staining 
capacity  ; embedded  in  this  substance  are  numerous 
granules  of  a very  intensely  staining  material  which 
is  known  as  chromatin.  There  are  strong  reasons  for 
believing  that  the  chromatin  of  the  nucleus  is  of 
special  importance  from  the  point  of  view  of  the 
mechanism  of  heredity.  This  reticular  structure  of 
the  nucleus  is  indicated  in  a diagrammatic  fashion  in 
Fig.  19. 

Further  light  is  thrown  upon  the  detailed  structure 
of  the  nucleus  by  the  changes  which  become  visible 
during  the  process  of  nuclear  division.  This  process, 
which  is  known  as  mitosis,  we  must  now  proceed  to 
describe. 

In  the  description  of  mitosis  which  follows,  the 
account  of  this  process  has  been  somewhat  generalized 
and  simplified,  and  Figs.  19  to  26,  which  illustrate  the 
phenomena,  are  purely  diagrammatic.  It  is  hoped 
that  the  most  important  features  of  this  complicated 
process  may  be  in  this  way  rendered  comprehensible ; 
and  although  in  different  organisms  considerable 
variations  in  the  details  of  the  process  are  to  be  met 
with,  yet  in  their  general  features  all  ordinary  mitoses 
in  animals  and  plants  commonly  coiSbrm  to  the 
essential  type  of  our  description. 

The  first  change  in  the  appearance  of  the  nucleus 
which  indicates  that  a division  is  about  to  take  place 
consists  in  a rearrangement  of  the  chromatin  network, 
which  now  takes  on  the  appearance  of  a tangled 
thread  (Fig.  20).  The  outwardly-directed  loops  of 
this  skein  often  correspond  to  the  separate  portions 


CHROMOSOMES 


24Q 


into  which  the  thread  eventually  breaks  up.  The 
thread  gradually  grows  shorter  and  thicker,  and 
presently  becomes  divided  into  a number  of  pieces 
which  are  known  as  chromosomes.  In  the  chromo- 
somes the  shortening  and  thickening  process  is  con- 
tinued until  these  bodies  arrive  finally  at  the  form  of 
stumpy  rods,  each  of  which  often  becomes  bent  into 


the  form  of  a horseshoe.  Meanwhile  the  nuclear 
membrane  breaks  down,  so  that  the  hyaline  substance 
of  the  nucleus  becomes  continuous  with  that  of  the 
cell  body  surrounding  it.  A fresh  phenomenon  now 
becomes  visible.  A spindle-shaped  arrangement  makes 
its  appearance  consisting  of  a nmnber  of  minute  fibrils 
which  connect  together  two  points — the  poles  of  the 
spindle  — situated  at  opposite  ends  of  the  cell.  The 


250 


RECENT  CYTOLOGY 


chromosomes  now  change  their  position  so  that  they 
come  to  lie  in  the  plane  of  the  equator  of  the  spindle, 
and  about  this  time,  but  sometimes  earlier,  each 
chromosome  splits  longitudinally  into  two  equal 
portions  (Figs.  22,  23).  This  splitting  in  the  case  of 
each  chromosome  takes  place  in  the  equatorial  plane 
of  the  spindle,  so  that  one  member  of  each  pair  of 
daughter  chromosomes  faces  towards  one  pole  of  the 
spindle,  and  the  second  towards  the  other  pole.  The 
members  of  each  pair  of  daughter  chromosomes  now 
begin  to  move  away  from  one  another  towards  the' 
two  poles  of  the  spindle,  and  as  they  do  so  the  first 
indication  of  a dividing  wall  between  the  two  new 
cells  begins  to  make  its  appearance  in  the  equatorial 
plane. 

Arriving  at  the  poles,  the  daughter  chromosomes 
begin  to  elongate,  and  to  put  out  processes  which 
finally  meet  and  fuse  with  those  of  their  neighbours 
to  form  the  chromatin  reticulum  of  the  new  nuclei 
(Fig.  25).  Surrounding  each  new  nucleus,  thus 
developing  at  either  pole  of  the  now  rapidly  dis- 
appearing spindle,  a new  nuclear  membrane  makes  its 
appearance  ; the  dividing  wall  in  the  position  of  the 
equator  of  the  spindle  develops  into  a complete 
partition  (at  least  in  the  case  of  plants,  in  which, 
however,  a number  of  minute  passages  are  left  pene- 
trating the  cell  wall  and  preserving  the  communication 
between  the  protoplasmic  contents  of  the  separate 
cells) ; and  the  division  into  two  new  cells  is  thus 
completed  (Fig.  26).  Each  new  cell  is  provided  with 
a nucleus  into  which  has  entered  precisely  its  fair 


CHROMOSOMES 


251 

share  of  the  chromatin  which  was  present  in  the 
parent  nucleus. 

A great  deal  of  evidence  has  recently  accumulated 
to  show  that  chromosomes  are  very  definite  and 
important  organs.  In  the  first  place,  the  number  of 
chromosomes  which  make  their  appearance  at  each  cell 
division  is  the  same  in  all  the  cells  of  any  given 
creature,  and  this  numerical  constancy  further  extends 
to  the  cells  of  all  the  members  of  a particular  species, 
though  in  members  of  allied  species  the  number  of 
chromosomes  may  be  different.  In  widely  separated 
species  the  number  of  chromosomes  varies  consider- 
ably; thus  from  2 to  200  have  been  counted  in  the 
case  of  various  different  members  of  the  animal  and 
vegetable  kingdoms.  One  of  the  commonest  numbers 
found  Is  twelve,  and  this  number  occurs  in  a con- 
siderable variety  of  different  animals  and  plants. 

Next  it  has  been  shown  that  the  chromosomes 
which  arise  at  the  beginning  of  a nuclear  division  are 
identical  with  those  daughter  chromosomes  of  the 
preceding  division  which  originally  entered  into  the 
nucleus  now  about  to  divide.  An  example  of  the 
kind  of  evidence  upon  which  this  conclusion  is  based 
may  next  be  given. 

Figs.  27,  28,  and  29  show  the  three  possible  arrange- 
ments of  the  four  chromosomes  which  are  found  in  the 
cells  of  the  worm-like  animal  Ascaris,  as  seen  from  the 
direction  of  the  pole  of  the  spindle  in  the  dividing 
nucleus.  Of  these  arrangements,  that  shown  in 
Fig.  29  is  much  the  least  common.  Now  in  this  par- 
ticular case  the  chromosomes,  when  they  first  mak« 


RECENT  CYTOLOGY 


253 


their  appearance  immediately  before  the  process  of 
division,  are  found  with  their  extremities  situated  in 
little  pockets  or  bulgings  of  the  nuclear  membrane, 
so  that  their  exact  position  is  very  definitely  marked ; 
and  the  arrangement  of  the  chromosomes  may  be  any 
one  of  those  already  indicated.  Boveri  observed  that 


Fig.  27.  Fig.  28.  Fig.  29. 


in  the  case  of  two  neighbouring  cells  which  had 
originated  by  the  division  of  the  same  mother-cell,  the 
chromosomes  made  their  appearance  in  both  cases  in 
the  uncommon  position  of  Fig.  29.  Figs.  30  and  31 
indicate  their  actual  arrangement.  The  conclusion  to 
be  drawn  from  this  observation  is  that  the  same 


Fig.  30. 


Fig.  31. 


chromosomes  have  preserved  their  individuality  right 
through  the  resting  stage  of  the  nucleus,  to  reappear 
in  the  same  position  at  the  outset  of  a new  phase  of 
division. 

If  is  believed,  then,  that  the  same  stages  which  the 
chromosomes  passed  through  at  the  close  of  one 


CHROMOSOMES 


253 


nuclear  division,  giving  rise  to  the  nuclear  reticulum  in 
the  daughter  nucleus,  are  repeated  in  the  reverse  order 
at  the  outset  of  the  next  division  ; the  same  processes 
are  withdrawn  into  the  same  chromosomes,  and  these 
shorten  into  structures  identical  with  those  which 
passed  into  the  nucleus  at  its  first  formation,  except 
that  they  have  increased  in  bulk  during  the  interval.* 

Boveri,  in  fact,  concludes  that  the  separate  chromo- 
somes are  to  be  looked  upon  as  distinct  individuals — 
almost  as  separate  simple  organisms— which  preserve 
their  individuality  throughout  the  history  of  the  cell, 
and  reproduce  themselves,  just  as  cells  and  nuclei  do, 
by  a process  of  bipartition.  As  far  as  the  chromo- 
somes themselves  are  concerned,  their  typical  or 
resting  form  is  that  of  the  short  simple  rods  seen  in 
mitosis.  The  branched  anastomozing  character  seen 
during  the  stage  of  the  nuclear  reticulum  is  associated 
with  the  active  co-operation  of  the  chromosomes  in  the 
physiological  processes  going  forward  within  the 
nucleus.  For  this  reason  the  term  ‘ resting  stage  ’ 
applied  to  this  condition  of  the  nucleus  is  a particularly 
inappropriate  one. 

Boveri  illustrates  the  amount  of  credence  which  he 
would  attach  to  this  theory  of  the  individual  persis- 
tence of  the  chromosomes  throughout  the  resting 
condition  of  the  nucleus,  by  means  of  the  following 
analogy  ; ‘ We  make  water  from  oxygen  and  hydrogen, 
and  from  this  water  we  can  obtain  oxygen  and  hydrogen 
again  in  the  same  proportions.  Just  in  the  same  way 
as  the  chemist  on  the  evidence  of  these  facts  regards 
*[A  number  of  similar  examples  have  been  described  since  this 
paragraph  was  written.] 


254 


RECENT  CYTOLOGY 


water  as  containing  oxygen  and  hydrogen,  although 
the  properties  of  these  substances  are  completely  m 
abeyance,  so  1 believe  it  to  be  with  equally  good 
reason  that  our  theory  regards  the  individual  chromo- 
somes as  being  presented  in  the  resting  nucleus.’’'^ 

Since  Boveri  expressed  this  opinion  Rosenberg  has 
produced  further  evidence  of  an  equally  convincing 
kind.  He  finds  that  in  the  case  of  certain  plants  the 
chromosomes  do  not  pass  over  mto  a continuous 
reticulum  during  the  resting  condition  of  the  nucleus, 
but  remain  separate,  so  that  the  same  number  of 
chromatic  bodies  can  be  coimted  during  this  stage  as 
during  the  actual  process  of  mitosis. 

Boveri  has  also  produced  evidence  to  show  that 
different  chromosomes  play  different  parts  in  the 
economy  of  the  organism.  For  example,  when  dif- 
ferent chromosomes  were  artificially  removed  from 
the  nucleus  of  an  embryonic  cell  by  taking  advantage 
of  certain  abnormal  methods  of  division,  the  embryos 
which  arose  from  these  cells  • developed  to  different 
extents  and  in  different  abnormal  ways. 

This  result  is  of  particular  interest,  because  it  gives 
full  corroboration  to  the  suspicion,  previously  enter- 
tained, that  the  chromosomes  are  specially  concerned 
with  hereditary  processes — with  the  building  up  of 
particular  parts  of  the  developing  organism  into  shapes 
which  resemble  those  of  the  corresponding  parts  dis- 
played by  other  members  of  the  same  species ; and  it 
seems  further  to  show  that  particular  chromosomes 

* Dr.  T.  Boveri,  ‘ Ergebnisse  fiber  die  Konstitution  der 
Chromatischen  Substanz  des  Zellkems,’  p.  22. 


CHROMOSOMES  255 

may  be  specially  concerned  in  the  development  of 
particular  parts. 

Sutton  and  others  have  shown  that  the  different 
cl  .romosomes  contained  in  the  same  nucleus  of  a partic- 
ular animal  may  be  of  different  shapes  and  sizes,  so 
th'at  each  is  individually  recognisable.  It  is  thus  pos- 
sible to  demonstrate  that  an  identically  similar  set  of 
chromosomes  appear  at  each  of  several  successive  cell 
divisions.  In  this  way  additional  evidence  is  afforded 
of  the  individual  persistence  of  the  chromosomes  and 
of  their  separate  identity. 

We  have  already  pointed  out  how,  in  the  process  of 
fertilization,  the  two  conjugating  germ-cells,  as  well 
as  the  nuclei  which  they  contain,  become  completely 
fused  together  to  form  a single  cell  containing  only  one 
nucleus.  It  might  have  been  expected  that  the  sepa- 
rate chromosomes  contained  in  the  conjugating  nuclei 
would  also  fuse  together  in  pairs  during  this  process,  but 
this  is  not  the  case.  The  paternal  and  maternal  chromo-  - 
somes  remain  separate,  so  that  the  nucleus  of  the  zygote 
contains  twice  as  many  chromosomes  as  does  that  of 
either  of  the  gametes  by  the  fusion  of  which  it  arose. 
This  double  number  of  chromosomes  reappears  at  every 
cell  division  during  the  embryonic  history  of  the 
zygote,  and  thus  the  fact  is  accounted  for  that  [with 
certain  exceptions  to  be  mentioned  later  (p.  272)] 
the  number  of  chromosomes  in  a somatic  nucleus  is 
always  even.  Thus  we  see  that  the  chromosomes 
derived  from  the  two  parents  are  present  side  by  side  in 
the  nuclei  of  the  offspring,  and  reproduce  themselves  by 
bipartition  at  every  nuclear  division  which  takes  place 


RECENT  CYTOLOGY 


256 

in  the  zygote.  In  this  way  every  somatic  nucleus  of 
the  latter  contains  a double  set  of  chromosomes,  half 
of  them  being  descended  from  the  chromosomes  intro- 
duced by  one  parent,  whilst  the  other  half  came  from 
the  second  parent. 

There  is  reason  to  believe  that  the  set  of  chromo- 
somes derived  from  one  parent  is  complete  in  itself, 
containing  everything  necessary  for  the  development 
of  a normal  individual.  Indeed,  in  some  cases  of 
parthenogenesis  (development  of  the  unfertilized  egg), 
egg  cells  have  been  known  to  develop  which  contained 
only  a single  set  of  chromosomes.  Boveri  proved 
very  prettily  that  the  paternal  set  of  chromosomes  is 
equally  adequate  for  complete  development.  By  dint 
of  violent  shaking  Boveri  contrived  to  remove  the 
nucleus  from  the  egg-ceUs  of  a sea-urchin,  and  he 
afterwards  allowed  a sperm-nucleus  to  enter  the 
enucleated  egg,  which  presently  developed  into  a 
complete  embryo.  Thus  it  was  shown  that  the 
paternal  as  well  as  the  maternal  set  of  chromosomes 
is  sufficient  by  itself  to  determine  the  proper  production 
of  all  the  organs  of  the  embryo.  But  Boveri  also 
showed  that  if  any  chromosome  of  the  paternal  (or 
maternal)  set  were  wanting  in  such  a case,  normal 
development  of  the  embryo  could  no  longer  take  place. 
Let  it  once  more  be  emphasized  that  the  somatic  cells 
of  an  ordinary  organism  contain  a double  complement 
of  essential  nuclear  material. 

Since  the  gametes  contain  only  half  as  many  chromo- 
somes as  the  somatic  cells,  and  since  the  number  of 
chromosomes  present  in  the  latter  is  constant  for  each 


CHROMOSOMES 


257 


species,  it  follows  that  either  during  the  formation  of 
the  gametes,  or  at  some  one  or  other  of  the  cell 
divisions  leading  up  to  their  formation,  there  must  occur 
a reduction  in  the  number  of  chromosomes  to  one-half 
of  their  former  number.  In  the  case  of  the  higher 
animals  this  reduction  takes  place  during  the  two  cell 
divisions  which  directly  lead  up  to  the  formation  of  the 
gametes  themselves.  In  plants,  on  the  other  hand,  the 
reduction  takes  place  during  the  formation  of  those  cells 
which  are  known  as  spores.  From  these,  after  a certain 
number  of  intervening  cell  generations,  the  gametes 
take  their  origin.  These  intervening  cell  divisions  in 
plants  are  characterized  in  every  case  by  the  appear- 
ance of  the  reduced  number  of  chromosomes.  In 
the  higher  plants,  in  fact,  a generation  is,  as  it  were, 
interposed  between  the  reducing  division  and  the 
actual  formation  of  the  gametes.  For  the  spores  are 
themselves  unicellular  reproductive  bodies  like  the 
gametes,  but  differ  from  the  latter  in  the  fact  that 
they  develop  without  undergoing  conjugation,  and 
give  rise  to  a larger  or  smaller  mass  of  tissue  consisting 
of  cells  with  the  reduced  number  of  chromosomes. 
From  the  fact  that  the  cells  of  this  gamete-bearing 
generation  contain  half  as  many  chromosomes  as 
those  of  the  spore-bearing  generation  with  which  it 
alternates,  the  generation  produced  from  the  spores 
has  been  spoken  of  as  the  A;-generation  in  contrast 
with  the  ordinary,  or  2X-,  generation.  In  animals  the 
Af-generation  is  reduced  to  a single  generation  of  cells 
only,  which  is  represented  by  the  gametes  themselves. 

We  must  next  proceed  to  examine  the  actual 

17 


RECENT  CYTOLOGY 


258 

method  by  which  the  reduction  in  the  number  of  the 
chromosomes  is  brought  about. 

The  simplest  type  of  the  process  of  reduction  of  the 
chromosomes  takes  place  at  the  formation  of  the  male 
germ-ceUs,  or  spermatozoa,  of  animals.  For  the  sake  of 
clearness  we  shall  consider  the  case  of  an  animal  in 
which  the  somatic  cells  contain  four  chromosomes  only, 
and  in  which  the  reduced  number  characteristic  of  the 
gametes  is  therefore  two. 

The  reduction  in  number  of  the  chromosomes  takes 
place  during  two  successive  cell  divisions  which 
immediately  lead  up  to  the  formation  of  the  germ- 
cells.  A particular  mother-cell  divides  twice  in  rapid 
succession,  and  the  four  cells  thus  arising  develop  into 
spermatozoa  without  further  subdivision.  During 
these  two  nuclear  divisions  the  somatic  number  of 
chromosomes  becomes  halved,  giving  rise  to  the 
number  characteristic  of  the  gametes. 

Immediately  before  the  first  of  these  divisions  the 
chromosomes  become  closely  associated  together  in 
pairs,  and  in  certain  cases  it  has  been  shown  that  one 
member  of  each  pair  is  very  probably  the  descendant 
of  a chromosome  derived  from  the  male  parent,  whilst 
the  other  member  of  the  pair  is  the  descendant  of  the 
corresponding  maternal  chromosome. 

This  association  of  the  chromosomes  in  pairs  may 
be  so  close,  and  may  take  place  so  early,  that  when 
these  bodies  are  first  visibly  differentiated  only  half 
the  usual  nmnber  of  them  is  to  be  seen.  But  in  these 
cases,  too,  it  is  reasonable  to  believe  that  each  of 
the  chromosomes  actually  visible  consists  of  a maternal 


THE  REDUCING  DIVISION 


259 


and  a paternal  member  fused  together.  Each /of  the 
visible  chromatic  bodies  next  divides  into  four  parts, 
the  set  of  four  deeply  staining  bodies  being  known  as  a 
tetrad.  Thus  when  there  are  four  somatic  chromosomes 
the  number  of  tetrads  appearing  will  be  two  (Fig.  32). 
A mitosis  now  takes  place,  during  which  there  is  no 
further  division  of  chromosomes,  but  half  of  each 


tetrad  passes  to  either  pole  of  the  nuclear  spindle, 
so  that  each  daughter  nucleus  comes  to  contain  two 
half-tetrads,  each  consisting  of  a pair  of  deeply-staining 
bodies  (Fig.  34).  This  division  is  not  followed  by  the 
production  of  a resting  nucleus,  for  before  any  nuclear 
reticulum  is  formed,  and  while  the  half-tetrads  still 
retain  their  definite  appearance,  the  datighter  nuclei 
divide  again.  At  this  second  division  in  each  nucleus 

17—3 


26o 


RECENT  CYTOLOGY 


the  separate  members  of  each  of  the  two  half-tetrads 
pass  to  opposite  poles  (Figs.  35,  36).  In  the  nucleus 
of  each  of  the  four  cells  which  thus  arise  there  is,  there- 
fore, present  one  quarter  of  each  of  the  four  chromo- 
somes which  originally  appeared — one  member,  that 
is  to  say,  of  each  tetrad  (Fig.  37).  Each  of  the  cells  of 
which  we  have  thus  traced  the  origin  develops  directly 
into  a single  spermatozoon. 


The  method  of  development,  or  maturation,  of  the 
ova,  or  egg-cells,  of  animals  is  in  all  essential  respects 
similar  to  the  process  by  which  the  spermatozoa  arise. 
It  differs,  however,  in  the  fact  that  of  the  four  cells 
which  result  from  the  corresponding  divisions,  one  is 
very  large  and  constitutes  the  ovum,  whilst  the  other 
three  are  very  minute,  and  are  apparently  of  no  further 
importance.  In  the  accompanying  diagrams  (Figs.  40 
to  42),  the  smaller  cells,  or  polar  bodies,  have  been 


THE  REDUCING  DIVISION 


261 


enormously  exaggerated  relatively  to  the  size  of  the 
ovum  itself. 

The  original  tetrad  is  believed  in  all  cases,  and  has 
been  actually  observed  in  a few  cases,  to  arise  by  a 
separation  of  the  two  fused  chromosomes,  followed  by 
a division  of  each  of  these  bodies  into  two.  In  cases 
where  the  chromosomes  retain  their  rod-like  appear- 
ance throughout  these  changes  there  would  seem  to  be 
some  doubt  as  to  whether  the  first  of  the  divisions 
giving  rise  to  the  ‘ tetrad  ’ is  transverse  or  longitudinal 
in  direction,  and  it  is  possible  that  the  process  may  be 
different  in  different  cases.  But  it  is  generally  agreed 
that  the  first  division  separates  the  two  original 
chromosomes,  and  that  at  the  first  of  the  two  nuclear 
divisions  which  ensue  the  members  of  a pair  of  parental 
chromosomes  pass  into  separate  nuclei.  The  second 
division,  on  the  other  hand,  like  an  ordinary  mitosis, 
separates  halves  of  chromosomes.  This  agreement 
among  authorities  is  explained  by  the  circumstance 
that  those  observers  who  have  seen  a longitudinal 
first  division  believe  that  the  parental  chromosomes 
conjugated  side  by  side,  whilst  those  who  describe  a 
transverse  division  describe  also  an  end-to-end  con- 
jugation of  the  chromosomes. 

The  first  of  these  two  ideas  is  the  one  illustrated  in 
the  accompanying  diagram  (Fig.  43),  representing  the 
behaviour  of  a single  pair  of  parental  chromosomes 
during  the  two  nuclear  divisions  which  give  rise  to 
four  sperm  cells.  The  chromosome  derived  from  one 
parent  is  shaded,  whilst  the  other  is  left  blank. 

Thuj>  the  first  of  the  two  gamete-producing  divisions 


263 


RECENT  CYTOLOGY 


differs  from  all  other  mitoses  in  the  fact  that  in  it  an 
actual  separation  of  whole  chromosomes  takes  place  ; 
it  is  a qualitative  and  not  only  a quantitative  division. 
It  is  to  this  mitosis  that  the  term  reducing  division  is 
properly  applied. 

We  have  to  notice  that  at  one  stage  of  the  process 
now  described  the  chromosomes  derived  from  the  two 
parents  are  in  a close  state  of  fusion.  It  would  seem 
as  if  the  actual  conjugation  of  chromosomes,  which 
failed  to  take  place  when  the  conjugating  gametes  and 
their  nuclei  fused  together  in  the  formation  of  the 
zygote,  was  only  delayed,  and  now  occurs  hundreds  or 


thousands  of  cell  generations  after  the  actual  process 
of  fertilization,  and  immediately  before  the  production 
of  those  cells  which  are  to  give  rise  to  the  new 
individual. 

It  may  be  pointed  out  that,  although  the  chromo- 
somes which  emerge  from  this  fusion  seem  to  be 
identical  with  those  which  entered  into  it,  yet  it  is 
difficult  to  believe  that  they  have  not  undergone  some 
change,  or  exercised  some  mutual  influence  upon  one 
another.  If  no  such  influence  has  been  exerted,  it  is 
difficult  to  imagine  any  possible  reason  for  the  process 
of  fusion  taking  place  at  all. 

In  the  higher  plants  a similar  reducing  division 


THE  INDUCING  DIVISION  263 

takes  place  at  the  formation  of  the  spores,  which  arise 
in  sets  of  four,  each  set  corresponding  to  a group 
of  four  spermatozoa,  or  to  the  ovum  and  the  three 
polar  bodies  of  an  animal.  In  the  case  of  flowering 
plants  the  nuclei  contained  in  the  spores  make  a few 
further  divisions,  at  each  of  which  the  reduced  number 
of  chromosomes  is  to  be  observed,  and  one  or  more  of 
the  cells  thus  finally  produced  take  on  the  character  of 
germ-cells.  The  spores  are  of  two  kinds,  large  and 
small,  the  latter  being  the  pollen  grains.  The  larger 
spores  give  rise  to  female  gametes  and  the  smaller  to  male, 
and  fertilization  takes  place  in  the  ordinary  manner  by 
a fusion  between  the  nuclei  of  these  germ-cells. 

We  have  seen  so  far  that  the  number  of  chromosomes 
contained  in  the  somatic  nuclei  of  a given  species  is 
always  the  same,  and  is  always  even.*  We  have  also 
seen  that  this  number  is  made  up  of  two  separate  sets 
derived  respectively  from  the  two  parents,  and  that 
the  members  of  the  two  sets  preserve  their  separate 
individuality  right  through  the  long  series  of  nuclear 
divisions  which  take  place  during  the  development  of 
the  individual  .zygote.  A fusion  of  chromosomes  of 
paternal  and  maternal  origin  respectively  takes  place 
only  in  the  direct  line  of  ancestry  of  the  germ-cells 
which  are  destined  to  give  rise  to  new  members  of  the 
species.  This  process  of  fusion  takes  place  in  animals 
immediately  before  the  formation  of  the  actual  germ- 
cells,  but  in  plants  a larger  or  smaller  number  of  ceU 
generations  earlier.  After  fusion  the  paternal  and 
maternal  chromosomes  apparently  separate,  and  the 
• Except  in  cases  such  as  are  described  on  pp.  270-276. 


264 


RECENT  CYTOLOGY 


nuclear  division  which  ensues  differs  from  all  other 
mitoses  in  the  fact  that  instead  of  merely  dissevering 
halves  of  chromosomes,  the  actual  somatic  chromo- 
somes separate  and  become  distributed  equally  between 
the  resulting  nuclei ; so  that  in  these  nuclei,  and  in  the 
germ  nuclei  which  arise  by  their  division,  the  number 
of  chromosomes  is  reduced  to  half  the  somatic  number. 
When  fertilization  takes  place  the  somatic  number  of 
chromosomes  is  restored  by  the  union  of  nuclei,  each 
of  which  contains  half  that  number. 

Is  it  possible  to  throw  any  further  light  upon  the 
meaning  of  these  facts  regarding  the  behaviour  of  the 
minute  constituent  parts  of  organisms  ? 

Let  us  return  to  Mendel’s  experimental  discovery, 
of  which  an  account  was  given  in  the  last  two  chapters, 
and  let  us  consider  the  case  of  a cross  between  parents 
which  differ  in  respect  of  two  pairs  of  allelomorphs. 
Expressing  these  pairs  as  A-a  and  B-h,  Mendel 
showed  that  the  germ-cells  of  the  cross-bred  or  hetero- 
zygote bear  in  equal  numbers  the  combinations  ^5,  Ab, 
aB,  and  ab.  Now,  it  seems  clear  from  this  behaviour 
that  the  allelomorphs  must  be  represented  in  the  cells 
of  the  organism  by  some  kind  of  definite  particles, 
which  remain  distinct  from  one  another  throughout 
aU  the  cell  divisions  of  the  body,  since  we  know  that 
at  the  formation  of  the  germ-ceUs  these  characters  are 
capable  of  becoming  completely  segregated.  Let  us, 
then,  trace  the  behaviour  of  the  allelomorphs  in  a 
diagrammatic  way,  regarding  each  as  a distinct  par- 
ticle. These  particles  we  may  distinguish  by  certain 


AND  MENDEL’S  LAW 


265 


letters.  A and  a are  the  allelomorphs  of  one  pair, 
B and  b those  of  the  other,  and  we  will  suppose  that 
one  of  the  parents  exhibits  the  characters  A and  b 
and  the  other  the  characters  a and  B (Fig.  44).  Then, 
in  the  zygote  resulting  from  fertilization.  A,  a,  B,  and  b 
will  aU  be  present. 

Since  all  the  cells,  at  least  in  the  direct  line  of 
ancestry  of  the  gametes,  must  contain  every  allelo- 
morph, it  will  be  necessary  for  the  particle  representing 
each  allelomorph  always  to  divide  into  two  before  a 
ceU  division  takes  place,  for  only  in  this  way  can  some- 
thing corresponding  to  each  allelomorph  pass  into  each 
of  the  two  cells  produced  by  the  division.  And  a 
similar  process  will  be  repeated  at  each  somatic 
mitosis  (Fig.  44).  At  the  formation  of  the  germ-cells, 
however,  or  at  some  preceding  cell  division,  the  two 
members  of  each  pair  of  allelomorphs  must  become 
separated  from  one  another  in  such  a way  that  the 
particles  originally  derived  from  different  parents 
pass  over  into  different  cells.  When  two  pairs  of 
allelomorphs  are  concerned,  this  process  of  separation 
can  take  place  in  either  of  the  two  ways  shown  in 
Fig,  45.  And  the  experimental  evidence  shows  that 
the  two  methods  occur  with  equal  frequency  in  the 
formation  of  the  germ-cells  of  the  same  heterozygote. 

Anyone  who  has  succeeded  in  following  the  above 
account  of  the  behaviour  of  the  supposed  particles 
representing  Mendelian  allelomorphs  in  the  cells  of  a 
hybrid  organism,  on  comparing  it  with  the  preceding 
description  of  the  behaviour  of  chromosomes  in  the 


266 


RECENT  CYTOLOGY 


AND  MENDEL’S  LAW 


267 


somatic  and  reducing  divisions  respectively,  can 
scarcely  fail  to  be  struck  by  the  extraordinary  simi- 
larity between  the  two  processes.  It  seems  quite 
clear  that  there  must  be  some  real  connection  between 
the  behaviour  of  chromosomes  as  seen  microscopically 
on  the  one  hand,  and  the  behaviour  of  allelomorphic 
characters  as  deduced  from  the  results  of  experiment 
on  the  other  ; and  that  the  evidence  derived  from  these 
two  forms  of  study  is  bound  to  be  of  considerable 
mutual  benefit. 

At  first  sight  it  might  be  thought  that  the  chromo- 
somes are  the  actual  bearers  of  Mendehan  characters, 
in  the  sense  that  each  chromosome  represents  a single 
allelomorph ; and,  indeed,  there  is  no  fundamental 
difference  between  the  behaviour  of  chromosomes  and 
that  of  our  supposed  character-bearing  particles.  But 
there  is,  at  least  in  some  cases,  a fatal  objection  to 
this  belief  in  the  fact  that  in  certain  plants  the  number 
of  separate  allelomorphic  pairs  which  may  be  bom  by 
a hybrid  is  greater  than  the  reduced  number  of  chromo- 
somes which  the  gefm-eeUs  of  this  hybrid  contain. 
For  instance,  in  the  case  of  the  pea  the  reduced 
number  of  chromosomes  is  seven,  and  Mendel  himself 
described  the  behaviour  of  seven  independent  pairs  of 
allelomorphs  in  peas.  Recent  study  has  revealed  the 
presence  of  at  least  four  additional  pairs  of  allelo- 
morphs in  these  plants,  aU  of  which  are  probably  equally 
independent  of  one  another. 

We  must,  therefore,  seek  a different  explanation, 
and  de  Vries  has  recently  suggested  one  which  up  to 


268 


RECENT  CYTOLOGY 


the  present  time  appears  the  most  likely  to  represent 
the  true  account  of  the  phenomena.  De  Vries’  ex- 
planation is  associated  with  the  finer  structure  of  the 
chromosomes  themselves,  a subject  upon  which  we 
have  not  hitherto  entered.  Under  high  powers  of  the 
microscope,  and  after  very  careful  preparation,  it  is 
possible  to  observe  that  each  chromosome  contains  a 
number  of  separate  darkly-staining  granules  which  are 
known  as  chromomeres.  When  the  pairs  of  parental 
chromosomes  fuse  together  previous  to  the  reducing 
division,  the  chromomeres  which  they  contain  appear 
to  meet  together  in  corresponding  pairs.  The  members 
of  each  pair  fuse  together  completely,  afterwards 
separating  as  the  chromosomes  separate; 


Fig.  46. 


De  Vries  supposes  the  Mendelian  allelomorphs  to 
be  contained  in  the  chromomeres,  and  that  when  these 
granules  fuse  together  an  exchange  of  allelomorphs 
takes  place  between  the  chromosomes.  This  ex- 
change proceeds  in  such  a way  that  when  the  chromo- 
somes separate  after  fusion,  it  is  a matter  of  simple 
chance  whether  a particular  allelomorph  has  remained 
in  the  chromomere  which  originally  contained  it,  or 
has  passed  over  into  the  other  member  of  the  pair. 
Thus,  in  a sufficient  number  of  cases  we  should  get 
aU  possible  chance  distributions  of  allelomorphs  be- 


CHROMOMERES 


269 


tween  the  two  chromosomes,  except  that,  of  course,  the 
two  members  of  the  same  pair  of  allelomorphs  would 
never  coexist  in  the  same  chromosome.  Since  the 
two  chromosomes  of  a pair  pass  into  different  germ- 
cells,  precisely  that  chance  distribution  of  allelomoi’phs 
which  is  required  on  the  Mendelian  theory  would  thus 
be  arrived  at. 

De  Vries’  explanation  throws  light  on  one  pheno- 
menon which  is  not  accounted  for  on  the  supposition 
that  each  chromosome  represents  a separate  allelo- 
morph. In  the  diagrams  previously  given  of  the 
behaviour  of  Mendelian  characters  within  the  cells  we 
have  given  no  indication  of  a conjugation  in  pairs 
previous  to  the  reducing  division.  Such  a process  of 
fusion  is,  however,  one  of  the  most  marked  phenomena 
in  the  behaviour  of  the  chromosomes  at  the  parallel 
stage  of  their  existence.  On  the  chromosome-allelo- 
morph view,  the  phenomenon  of  mitosis  as  bringing 
about  an  equal  division  of  hereditary  particles  between 
the  cells,  and  the  process  of  reduction  in  the  number 
of  the  chromosomes,  are  both  accounted  for,  but  there 
is  no  explanation  of  the  fusion  between  the  pairs  of 
chromosomes.  On  de  Vries’  view,  however,  this  process 
is  necessary  in  order  to  bring  about  the  necessary  redis- 
tribution of  allelomorphs  between  the  chromosomes,  and 
so  between  the  germ-cells  into  which  the  latter  pass. 

[More  recently  Professor  T.  H.  Morgan  has  brought 
forward  a much  more  complete  theory  of  the  relation 
between  chromomeres  and  Mendelian  characters,  based 
on  the  behaviour  of  characters  which  show  gametic 


270 


RECENT  CYTOLOGY 


coupling.  He  supposes  that  the  factors  for  all  the 
characters  of  a species  which  show  gametic  coupling 
with  one  another  are  borne  by  the  same  chromosome. 
In  the  Fruit-fly  {Drosophila)  there  is  rather  striking 
evidence  that  this  is  the  case,  for  there  are  four  groups 
of  coupled  characters,  which  may  be  called  A,  B,  C,  D. 
Any  character  in  one  group  is  more  or  less  closely 
coupled  with  any  other  character  in  the  same  group, 
but  quite  independent  of  characters  in  the  other 
groups.  Groups  A,  B,  and  C each  include  a number 
of  characters,  while  group  D is  very  small,  having  only 
two  or  three  characters  which  are  known  at  present. 
Now,  in  the  germ-cell  of  Drosophila  there  are  four 
chromosomes,  three  large  and  one  very  small,  and  the 
inference  is  therefore  made  that  each  chromosome 
corresponds  with  a group  of  coupled  characters. 
Further,  one  chromosome  is  an  ‘ idiochromosome,’  or 
sex-chromosome  (see  the  pages  immediately  following), 
and  all  the  characters  of  group  A are  ‘ sex-limited  ’ in 
transmission — i.e.,  they  are  transmitted  by  the  male 
only  to  those  offspring  which  also  receive  the  sex- 
chromosome.  The  correspondence  between  the  groups 
of  coupled  characters  and  of  the  chromosomes  in 
Drosophila  is  thus  so  close  as  to  make  the  hypothesis 
very  probable  that  the  chromosomes  actually  are  the 
bearers  of  Mendelian  characters.* 

* For  further  evidence  of  a different  kind  that  these  char- 
acters are  borne  by  chromosomes  in  Drosophila,  the  reader  is 
referred  to  C.  B.  Bridges'  paper,  ‘Non-Disjunction  as  Proof  of 
the  Chromosome  Theory  of  Heredity,*  in  Genetics^  voL  i.,  No.  i, 
January,  1916,  or  to  Morgan's  ‘Physical  Basis  of  Heredity,’  1919* 


CHROMOMERES  271 

The  problem  then  arises,  how  is  it  that  the  char- 
acters are  only  partially,  and  not  completely,  coupled  ? 
Morgan’s  answer  is  that  when  the  chromosomes  unite 
in  pairs  before  the  reduction  division  they  do  not 
simply  lie  side  by  side,  but  become  twisted  round  one 
another,  and  that  instead  of  untwisting  and  becoming 
separated  unchanged  before  the  nuclear  division,  they 
may  break  across,  so  that  what  was  part  of  one  chromo- 
some becomes  attached  to  what  was  part  of  the 
other. 

If  now  the  determiners  for  the  characters  are  arranged 
in  series  along  the  chromosome  like  the  chromomeres 
represented  in  Fig.  46,  when  two  determiners  are  placed 
close  together,  it  is  unlikely  that  the  break  will  occur 
between  them,  and  such  characters  will  be  closely 
coupled.  But  if  they  are  placed  farther  apart  the 
break  will  more  often  come  between  them,  and  they 
will  more  often  become  separated,  so  that  the  coupling 
is  less  strong.  And  finally,  if  the  determiners  are 
very  far  apart,  the  break  will  occur  equally  often  in 
such  a way  that  they  remain  associated  or  become 
separated,  and  in  such  a case  there  will  be  no 
coupling. 

It  should  be  mentioned  that  the  twisting  of  chromo- 
somes round  each  other  has  not  been  observed  in 
Drosophila,  which  is  not  well  suited  to  cytological  work, 
but  has  been  seen  in  other  animals  and  plants.] 

The  possibility  still  remains  that  in  certain  cases 
particular  characters  may  be  associated  with  par- 
ticular chromosomes  as  a whole,  and  we  shall  next 


272 


RECENT  CYTOLOGY 


proceed  to  describe  what  actually  seems  to  be  an 
example  of  this  sort.* 

The  case  we  have  to  describe  is  directly  concerned 
with  one  of  the  most  interesting  and  elusive  of  bio- 
logical problems — namely,  the  problem  of  the  heredity 
of  sex.  Until  recently  opinion  has  been  largely 
dominated  by  the  view  that  sex-production  is  in 
general  controlled  by  the  influence  of  external  con- 
ditions. A large  number  of  the  earlier  researches, 
and  some  of  the  later  ones,  have,  in  fact,  seemed  to 
show  that  sex  is  thus  determined.  The  most  recent 
knowledge  appears,  however,  to  point  conclusively  to 
the  belief  that  sex  is  already  determined  in  the  fer- 
tilized ovum.  The  fact  that,  so  far  as  the  evidence 
goes,  where  more  than  one  individual  develops  from 
the  same  fertilized  egg  all  are  of  the  same  sex  seems 
to  point  conclusively  in  this  direction;  and  further 
valuable  evidence  has  recently  been  adduced.  Professor 
E.  B.  Wilson  has  recently  investigated  the  behaviour 
of  the  chromosomes  in  the  somatic  cells  and  in  the 
germ-cells  of  a particular  species  of  insect  known  as 
Protenor  heljragi.  The  case  afforded  by  this  animal 
is  remarkable,  inasmuch  as  the  somatic  cells  in  the 
male,  and  only  in  the  male,  contain  an  odd  number 

* [In  the  last  few  years  definite  evidence  of  the  relation 
between  chromosomes  as  a whole  and  hereditary  characters 
has  been  obtained  in  several  plants;  e.g.,  the  lata  variety  of 
CEnothera  is  characterized  by  the  possession  of  an  extra 
chromosome,  and  the  variety  gigas  has  twice  the  normal 
number  (Gates).  Remarkable  observations  of  a similar  kind 
have  been  made  by  R.  P.  Gregory  in  Primula  sinensis  (Proc. 
Roy.  Soc.,  B.  87,  1914,  p.  484).] 


HEREDITY  OF  SEX 


273 


of  chromosomes.  An  irregularity  is  accordingly  intro- 
duced into  the  process  of  fusion  of  the  chromosomes 
in  pairs,  which,  as  already  described,  always  precedes 
the  formation  of  the  germ-cells  with  their  reduced 
number  of  chromosomes.  In  the  case  of  the  male 
Protenor  all  the  chromosomes  fuse  in  pairs  except  one, 
which  is,  of  necessity,  left  over.  This  odd  chromosome 
is  described  as  the  heterotropic  chromosome.  The 
female  Protenor  has  one  more  chromosome  in  its 
somatic  cells  than  the  male,  thus  making  up  an  even 
number;  that  is  to  say,  in  the  female  the  pair  to 
the  odd  chromosome  of  the  male  is  present,  so  that 
there  are  two  heterotropic  chromosomes,  or  idio- 
chromosomes.  These  fuse  and  separate  in  the  reducing 
division,  which  thus  proceeds  in  the  normal  manner 
in  this  sex.  In  the  male,  on  the  other  hand,  when 
the  reducing  division  occurs,  the  heterotropic  chromo- 
some passes  complete  into  one  of  the  resulting  cells. 
In  the  second  gamete -producing  division,  every  chro- 
mosome present  having  divided  into  two,  the  products 
of  this  division  pass  into  different  gametes.  These 
latter  divisions  are  of  two  kinds,  since  in  one  of  them 
the  heterotropic  chromosome  takes  part,  whilst  in  the 
other  it  is  wanting;  consequently,  two  out  of  the 
four  spermatozoa  eventually  produced  contain  the 
heterotropic  chromosome  and  two  do  not.  (Only  one 
spermatozoon  of  each  kind  is  shown  in  Fig.  47.)  Thus 
there  is  a differentiation  of  the  spermatozoa  into  two 
different  kinds,  and  one  of  these  kinds  contains  a 
chromosome  less  than  the  other.  On  the  other  hand, 

18 


274 


RECENT  CYTOLOGY 


every  egg  (as  well  as  every  polar  body)  contains  an 
idiochromosome . 

In  fertilization  some  of  the  eggs  become  impregnated 
by  spermatozoa  containing  the  heterotropic  chromo- 
some. Such  eggs  invariably  develop  into  females 
having  a pair  of  idiochromosomes  in  each  somatic 


ov,  ovum  ; ph,  polar  body ; sp,  spermatozoa  {a  and  b the  two  kinds). 


cell.  Other  eggs  are  fertilized  by  spermatozoa  lacking 
the  heterotropic  chromosome,  and  these  become  males, 
their  somatic  cells  containing  only  the  single  hetero- 
tropic chromosome  derived  from  the  egg.  The  ac- 
companying diagram  illustrates  the  behaviour  of  the 
chromosomes  during  these  processes.  The  hetero- 
tropic chromosomes  are  represented  as  black,  whilst 


HEREDITY  OF  SEX 


275 


the  remaining  chromosomes  are  left  white,  and  for  the 
sake  of  simplicity  only  two  pairs  of  the  latter  are 
indicated  in  the  somatic  cells. 

[Cases  like  that  of  Protenor,  in  which  the  male  has 
one  ‘ sex-chromosome  ’ and  the  female  two,  are  now 
known  in  a large  number  of  animals — for  example,  in 
many  Insects,  in  certain  Worms,  Molluscs,  etc.,  and  in 
several  Mammals,  including  (probably)  Man.]  The 
facts  render  it  scarcely  possible  to  doubt  that  there 
is  a causal  relationship  between  the  characteristics  of 
the  female  sex  and  the  presence  of  two  heterotropic 
chromosomes,  and  that  a similar  connection  exists 
between  maleness  and  the  presence  of  only  one. 

[Several  explanations  of  the  mechanism  of  sex-deter- 
mination by  means  of  these  ‘ sex-chromosomes  ’ have 
been  suggested.  It  was  originally  supposed  that  the 
two  chromosomes  of  the  female,  though  alike  in  appear- 
ance, were  different  physiologically,  one  bearing  a 
factor  for  femaleness  and  the  other  for  maleness,  and 
that  the  female  character  was  dominant.  The  hypo- 
thesis now  most  generally  accepted,  however,  is  that 
the  sex-chromosomes  are  aU  alike,  and  that  the  differ- 
ence between  the  two  sexes  is  produced  by  the  presence 
of  two  sex-chromosomes  in  the  female  and  one  in  the 
male.  The  case  would  then  be  comparable  with  that 
of  a pair  of  Mendelian  allelomorphs  in  which  the 
dominant  character  represents  the  presence  of  some- 
thing which  is  absent  in  the  recessive.  In  cases  of  the 
kind  here  considered,  femaleness  is  due  to  the  presence 
of  a determiner  (the  extra  chromosome)  which  is 


I 


276  RECENT  CYTOLOGY 

absent  in  the  male.  As  will  be  seen  later,  there  are 
other  cases  in  which  this  relation  is  reversed,  and  in 
which  the  male  has  a chromosome  which  is  lacking  in 
the  female.] 

In  various  other  insects,  some  of  them  closely  allied 
to  Protenor,  the  somatic  cells  of  the  male,  like  those  of 
the  female,  contain  each  a pair  of  idiochromosomes ; 
but  in  the  male  one  member  of  the  pair  is  much  larger 
than  the  other,  whilst  in  the  female  they  are  of  equal 
size.  The  behaviour  of  the  larger  member  of  the 
unequal  pair  of  chromosomes,  in  the  various  nuclear 
processes  which  occur  during  the  life-history,  is  pre- 
cisely like  that  of  the  single  heterotropic  chromosome 
of  Protenor.  It  is  still  possible  to  regard  this  chromo- 
some as  representing  a single  sex-chromosome,  and  to 
suppose  that  the  process  of  sex  determination  is  pre- 
cisely similar  in  the  two  cases.  On  this  supposition, 
the  smaller  idiochromosome  is  regarded  as  being  with- 
out function  so  far  as  sex  is  concerned. 

In  a third  group  of  insects,  some  nearly  related  to 
Protenor,  both  male  and  female  sexes  bear  alike  a pair 
of  idiochromosomes  of  equal  size.  Here,  again,  it  is 
possible  to  apply  the  same  theory  of  sex  determination 
by  simply  disregarding  one  of  the  idiochromosomes  of 
the  male  as  unimportant.-^  We  may  suppose,  in  fact, 
that  one  of  these  chromosomes  corresponds  to  the 
smaller  idiochromosome  of  the  preceding  case,  and  that 
it  takes  no  essential  part  in  these  phenomena.  The 
fact  that  this  chromosome  takes  no  active  part  in  these 
processes  may,  indeed,  have  led  to  its  reduction  in  the 


HEREDITY  OF  SEX 


277 


second  of  the  three  species,  and  to  its  final  disappear- 
ance in  the  first. 

Thus,  by  dint  of  a good  deal  of  speculation,  Wilson 
and  others  have  arrived  at  a possible  Mendelian  descrip- 
tion of  the  phenomenon  of  sex  in  a species  in  which 
the  chromosomes  of  male  and  female  are  alike;  and  it 
is  a description  which  has  its  basis  in  actual  phenomena 
observed  in  a number  of  related  animals.- 

[In  the  cases  considered  above,  the  female  has  two 
similar  idiochromosomes  and  the  male  either  only  one, 
or  two  which  are  unlike  one  another.  The  female  thus 
produces  eggs  which  are  all  alike,  while  the  male  pro- 
duces two  kinds  of  spermatozoa,  one  bearing  an  idio- 
chromosome  like  those  of  the  female,  the  other  either 
no  such  chromosome  or  a dissimilar  one.  Now,  in 
animals  belonging  to  groups  in  which  phenomena  of 
this  sort  have  been  observed,  it  has  also  been  observed 
that  there  are  certain  characters  which  are  transmitted 
by  the  male  only  to  his  daughters,  while  the  female 
transmits  them  impartially  to  her  offspring  of  both 
sexes.  The  best -known  example  of  this  sort  is  Droso- 
phila (see.41.  in  which  all  the  characters  of  group 
A are  transmitted  by  the  male  only  to  his  daughters 
(sex-limited  transmission  by  the  male).  In  Man  the 
same  sort  of  thing  occurs  in  several  abnormalities, 
notably  in  colour-blindness.  Colour-blindness  is  due 
to  the  loss  of  a factor  for  normal  vision,  and  this  factor 
for  normality  is  transmitted  by  a man  only  to  his 
daughters,  but  by  a woman  to  her  children  of  both 
sexes.  Hence  a colour-blind  man  married  to  a normal 


278 


RECENT  CYTOLOGY 


woman  has  normal  children,  while  a colour-blind  woman 
married  to  a normal  man  has  normal  daughters  and 
colour-blind  sons.  Colour-blind  men  are  frequent, 
because  the  daughters  of  colour-blind  men,  though 
normal  themselves,  transmit  the  affection  to  half  their 
sons;  but  colour-blind  women  are  very  rare,  because 
they  can  only  be  produced  by  the  marriage  of  a colour- 
blind man  to  a woman  who  is  herself  heterozygous  for 
the  affection. 

In  Moths  and  Butterflies,  and  also  in  Birds,  exactly 
the  converse  condition  is  found.  In  them,  instead  of 
the  male  transmitting  characters  only  to  his  daughters, 
it  is  the  female  which  transmits  certain  characters 
only  to  her  male  offspring,  while  the  male  transmits  to 
sons  and  daughters  alike.  In  Fowls,  for  example,  if  a 
barred  hen  is  mated  with  an  unbarred  cock,  all  the 
cockerels  will  be  barred  and  the  pullets  plain,  while  a 
cock  of  a barred  breed  mated  with  an  unbarred  hen 
produces  barred  chickens  of  both  sexes.  And  in  the 
Currant  Moth  {Abraxas  grossulariata) , in  which  this 
form  of  sex-limited  inheritance  was  first  investigated, 
it  has  been  shown  by  Doncaster  that,  at  least  in  certain 
strains,  the  female  has  a single  idiochromosome  and 
the  male  two — i.e.,  exactly  the  converse  condition  of 
that  found  in  Protenor,  etc.  In  this  case,  therefore, 
there  are  two  sorts  of  eggs,  male-producing  and  female- 
producing,  but  only  one  kind  of  spermatozoa,  and  it  is 
the  male-producing  eggs  (those  which  have  the  extra 
chromosome)  which  bear  the  sex-limited  character. 

These  facts,  therefore,  seem  to  prove  that  sex  is 


1 


HEREDITY  OF  SEX 


279 


inherited  as  a Mendelian  factor,  in  some  cases  female- 
ness, in  others  maleness,  being  due  to  the  presence  of  a 
factor  which  is  absent  in  the  other  sex;  and  the  whole 
category  of  characters  which  have  sex-limited  trans- 
mission appears  to  be  only  a special  case  of  gametic 
coupling,  due,  if  the  chromosome  theory  is  correct,  to 
the  association  of  the  factors  for  sex-limited  char- 
acters and  for  sex  itself  in  the  same  chromosome.] 

By  way  of  further  illustrating  the  far-reaching  im- 
portance of  the  information  which  has  been  rendered 
available  by  the  combined  use  of  experimental  and 
cytological  methods,  we  may  here  briefly  criticise  the 
celebrated  theory  of  inheritance  put  forward  by  Weis- 
mann  in  1892  under  the  name  of  the  ‘ Germ-Plasm 
Theory.’  Some  notice  of  this  theory,  which  might 
otherwise  have  been  permitted  to  go  the  way  of  similar 
valuable  provisional  hypotheses,  is  rendered  almost 
necessary  by  the  circumstance  of  its  having  been 
revived  in  a prominent  manner  in  the  English  transla- 
tion of  Weismann’s  book,  ‘ The  Evolution  Theory.’ 
In  this  book,  published  in  1904,  the  bearing  of  the 
Mendelian  evidence  upon  the  subject  of  inheritance  is 
practically  ignored ; although,  in  the  face  of  the  definite 
experimental  information  now  rendered  available, 
biologists  are  beginning  to  realize  that  the  circum- 
stantial evidence,  formerly  so  much  relied  upon,  will 
in  future  constitute  a much  less  prominent  feature  in 
these  discussions. 

Weismann’s  theory  of  inheritance,  and  the  Theory 
of  Ancestral  Heredity  in  its  original  form,  are  based 


28o 


RECENT  CYTOLOGY 


upon  a common  assumption,  which  is  now  shown  by 
Mendel’s  discovery  to  have  been  unfounded.  This  is 
the  assumption  that  all  ancestors  of  the  same  degree — 
e.g.,  grandparents — make  a substantially  equal  con- 
tribution to  the  hereditary  qualities  of  the  offspring. 
Mendel  has  shown  that  in  the  case  of  particular 
hereditary  characteristics  this  is  not  the  case. 

But  if  we  venture  to  criticise  Weismann’s  conception 
in  the  light  of  more  recent  knowledge,  it  must  not  be 
forgotten  that  biology,  and  especially  modern  cytology, 
owes  a great  debt  to  Weismann.  To  Weismann  is  due 
the  conception  of  the  isolation  of  the  germ-cells  from 
somatic  influences,  a view  which  is  in  complete  accord- 
ance with  the  Mendelian  view  of  the  inheritance  of 
definite  characters.  And  it  was  Weismann  who  first 
emphasized  the  belief  that  the  chromosomes  represent 
those  parts  of  the  nucleus  which  are  specially  concerned 
in  the  process  of  heredity.  These  conceptions— 
which,  indeed,  constitute  an  essential  part  of  his  own 
theory  of  heredity — have  stood  the  test  of  time  in  an 
admirable  manner. 

Let  us  turn  our  attention,  then,  for  a short  space  to 
the  Germ-Plasm  Theory  of  inheritance.  On  Weis- 
mann’s theory,  as  in  most  other  theories  of  heredity 
from  the  time  of  Darwin  and  Nageli  downwards,  the 
separate  parts  of  the  living  organism  are  supposed  to 
be  represented  by  separate  material  particles  in  the 
germ-cells.  These  representative  particles  are  known 
as  determinants.  A complete  set  of  determinants  in 
which  every  part  of  the  organism  is  thus  represented 


THE  GERM-PLASM  THEORY  281 

constitutes  an  id.  So  far  Weismann’s  hypothesis  is  in 
close  agreement  with  the  idea  of  representative  particles 
which  we  are  driven  to  adopt  by  the  facts  of  Mendelian 
inheritance,  except  that,  following  de  Vries,  we  should 
speak  of  separate  characters  rather  than  parts  as 
being  thus  represented ; for  there  seems  to  be  no  doubt 
that  the  same  character-determinant  can  affect  the 
development  of  a number  of  different  parts.  But  at 
the  next  step  the  Mendelian  parts  company  with 
Weismann.  The  latter  assumes  that  the  cells  of  an 
organism  contain  a large  number  of  ids,  or  complete 
sets  of  determinants,  half  of  the  total  number  being 
derived  from  either  parent,  and  that,  although  at  the 
reducing  division  which  precedes  the  formation  of  the 
gametes  the  total  number  of  ids  is  reduced  to  half  of 
what  it  was  in  the  somatic  cells,  still  several  ids 
derived  from  each  parent  are  present  in  every  germ- 
cell. 

Thus  the  reduced  number  of  chromosomes  in  the 
germ-cells  is  regarded  as  containing  all  the  primary 
constituents  of  both  parents.  And  it  is  an  essential 
point  in  Weismann’s  theory  that  he  regards  a given 
germ-cell  as  containing  a considerable  number  of  ids 
derived  from  its  ancestors,  all  near  ancestors  being 
thus  represented. 

But  Mendel’s  experiments  and  others  of  the  same 
kind  show,  in  the  case  of  a great  number  of  different 
characters,  that  although  every  essential  character  is 
represented  in  every  germ-cell,  yet  each  Mendelian 
character  is  represented  by  a paternal  or  a maternal 


282 


RECENT  CYTOLOGY 


determinant  only,  and  not  by  both.  Thus,  not 
only  are  all  immediate  ancestors  not  represented 
in  the  germ  - cells  in  respect  of  any  particular 
character,  but  only  one  of  the  parents  is  so  repre- 
sented— to  the  complete  exclusion,  so  far  as  we  can 
tell,  of  the  other  parent.  In  fact,  we  are  led  to 
believe  that  the  germ-cells  contain  one  set  of  de- 
terminants only — a single  id — whilst  the  somatic  cells 
contain  two  ids  only.  The  Mendelian  theory  is  thus 
seen  to  be  considerably  simpler  than  the  germ-plasm 
theory,  which  it  replaces.  At  the  same  time  it  must 
not  be  forgotten  that  many  of  the  conceptions  used  in 
the  Mendelian  expression  of  the  facts  are  borrowed 
from  Weismann’s  theory,  and  that  but  for  Weismann’s 
work  it  would  have  been  impossible  for  us  to  have 
got  so  far  in  the  co-ordination  of  the  facts  derived 
from  experiment  and  microscopic  observation  re- 
spectively. 

The  preceding  sketch  may  serve  to  show  how  Mendel’s 
observations  have  been  found  to  throw  light  upon  many 
of  the  facts  of  c3d;ology  the  meaning  of  which  was  pre- 
viously obscure;  and  how  it  affords  at  the  same  time 
a criterion  by  which  may  be  tested  the  truth  of  theories 
based  upon  the  interpretation  of  minute  phenomena 
only  made  visible  by  the  highest  powers  of  the  micro- 
scope. The  disinterment  of  Mendel’s  discovery  took 
place  in  1900 ; and  the  rapid  manner  in  which  the 
facts  of  cytology  have  been  found  to  fall  into  line  with 
Mendelian  conceptions  augurs  well  for  the  future 
progress  of  discovery  in  these  fields. 


THE  GERM-PLASM  THEORY 


283 


BIBLIOGRAPHY. 

Wilson,  E.  B.  : The  Cell  in  Development  and  Inheritance 
second  edition,  1900. 

,,  „ Recent  Researches  on  the  Determination 

and  Heredity  of  Sex.  Science,  January  8, 
1909. 

BoverI,  T.  : Ergebnisse  liber  die  Konstitution  der  chroma- 
tischen  Substanz  des  Zellkerns,  1904. 

Weismann,  a.:  The  Germ-Plasm:  a Theory  of  Heredity 
(English  translation  by  W.  N.  Parker  and  H.  Ronnfeldt), 

1893- 

Morgan,  T.  H.  : Heredity  and  Sex,  1914. 

,,  ,,  The  Mechanism  of  Mendelian  Heredity,  1915. 

„ „ The  Physical  Basis  of  Heredity,  1919, 

Doncaster,  L.  : The  Determination  of  Sex,  1914. 


CHAPTER  X 


EUGENICS 

Eugenics  is  defined  as  ‘ the  study  of  agencies  under 
social  control  that  may  improve  or  impair  the  racial 
qualities  of  future  generations,  either  physically  or 
mentally.’  Sir  Francis  Galton,  the  pioneer  of  this 
study,  may  well  be  regarded  as  one  of  the  foremost 
figures  in  recent  science.  His  claim  to  this  regard  is 
based  upon  a pre-eminence  in  many  different  branches 
of  knowledge — a claim  which  those  to  whom  his  work 
is  as  yet  unknown  will  find  substantiated  with  all 
becoming  modesty  in  his  ‘ Memories  of  My  Life. 
Much  of  Galton’s  work  in  other  fields  may  be 
forgotten,  overshadowed  as  it  is  by  his  foundation 
of  the  science  we  are  about  to  describe.  It  will  be 
for  future  generations  to  discover  that  niche  in  the 
temple  of  fame  which  is  fitting  for  the  man  who  first 
pointed  out  a feasible  direction  in  which  humanity 
might  set  about  its  own  improvement,  instead  of 
resting  content  in  the  vain  expectation  of  being  im- 
proved vicariously. 

We  have  neither  the  mathematical  equipment,  nor 
have  we  here  the  space,  to  enter  into  the  statistical 
inquiries  upon  which  Galton’s  conclusions  are  mainly 

284 


Francis  Galton. 


[To  face  p.  284. 


THE  LIBRARY 
Of  THE 

UNIVERSITY  OF  ILLINOIS 


GALTON’S  RESEARCHES 


285 


based.  For  an  account  of  these  the  reader  must  be  re- 
ferred to  the  admirable  popular  treatises  enumerated  at 
the  close  of  the  present  chapter.  In  spite  of  the  rather 
wide  divergence  of  these  studies  from  those  with  which 
this  book  is  mainly  concerned,  the  agreement  between 
the  conclusions  on  race-improvement  drawn  by  the 
students  of  Genetics,  on  the  one  hand,  and  by  those 
of  Biometry,  on  the  other,  is  a remarkable  one,  and 
may  perhaps  be  taken  to  indicate  that  both  these 
methods  are  right  in  their  several  directions. 

Consequently  we  propose  attempting  a summary 
of  the  line  of  researches  and  arguments  which  led 
Galton  to  his  conclusion  that  the  human  race 
is  capable  of  vast  improvements  in  physique,  in 
beauty,  in  character,  and  in  intellect.  The  importance 
of  this  conclusion  is  “augmented  by  the  corollary  that 
acquisition  of  these  improvements  leads  to  a keener 
appreciation  of  their  value,  and,  incidentally,  to  the 
greater  happiness  of  mankind.  But  the  student  of 
Eugenics  does  not  rest  satisfied  with  conclusions.  He 
proposes  to  utilize  the  great  forces  of  fashion  and 
public  opinion  as  agents  of  modification  and  improve- 
ment by  diverting  their  influence  into  the  right 
direction  and  out  of  their  present  remarkably  wrong 
direction. 

The  first  link  in  the  chain  of  evidence  was  forged 
long  ago  by  Darwin,  when  he  showed  how  far  man 
had  already  risen  from  a simpler  and  lower  type  of 
animal.  The  evolution  of  man  is  now  a part  of  the 
ordinary  intellectual  creed  of  most  educated  men,  and 
yet  few  politicians  or  charitable  people  pause  to  apply 


286 


EUGENICS 


this  belief  to  the  present  or  to  the  future.  Few 
realize  the  full  significance  of  the  inferences  that,  if 
man  has  risen  in  the  past,  he  may  still  rise  higher  in 
ages  yet  to  come ; if  the  type  has  undergone  modifica- 
tion in  the  past,  it  may  even  now  be  changing.  Those 
who  move  the  forces  of  money  or  of  popular  opinion 
should  take  heed  how  their  actions  affect  the  rate 
and  direction  of  this  possibly  momentous  change, 
i _ The  evolution  of  man,  like  that  of  other  animals, 

I is  believed  to  have  been  largely  due  to  the  effect  of 
\ selection.  Whether  the  variations  selected  were  great 
or  small  makes  little  difference  to  the  argument.  In 
former  days  this  selective  action  was  exerted  by  fierce 
competition,  which  led  to  the  survival  of  individuals 
endowed  with  certain  qualities  and  to  the  extinction 
of  other  individuals  differently  constituted. 

Modem  civilization  and  humanitarianism  have 
effectually  set  aside  the  action  of  natural  selection. 
The  result  at  the  present  day  indicates  that  the  upward 
progress  of  the  race  has  probably  almost  ceased,  and 
that  there  is  every  danger  of  finding  that  a downward 
journey  has  begun.  This  is  an  inevitable  conclusion 
to  those  who  accept  the  well-established  facts  (i)  that 
men  are  not  born  equal,  but  differ  widely  in  their 
birthright  in  respect  of  every  physical  and  mental 
character,  and  (2)  that,  although  every  man  is  free  to 
become  the  father  of  a family,  yet  the  tendency  to 
bring  up  large  families  is  becoming  relatively  smaller 
among  those  classes  of  society  which  we  must  regard 
as  the  best  endowed,  both  physically  and  mentally. 

Vi  Are  there  any  practicable  means  by  which  this 


PROGRESS  OF  THE  RACE 


287 


tfndency  to  degenerate  can  be  combated  and  changed 
into  an  upward  bias  ? In  order  to  discover  such  a 
means  it  is  needful  to  gather  all  possible  information 
with  regard  to  the  primary  factors  of  organic  evolution 
in  the  human  race — that  is  to  say,  variation  and 
heredity.  Galton  was  the  first  to  show  that  the  laws 
of  variation  and  heredity  in  man  are  closely  similar 
to  the  same  laws  in  other  organisips.  He  showed,  too, 
that  mental  and  moral  characters  are  inherited  just 
as  strongly  as  physical  characters — a fact  which  is 
by  no  means  so  generally  appreciated  as  it  should 
be,  in  spite  of  the  further  evidence  adduced  by  Prof. 
Karl  Pearson. 

Many  people  believe  that  the  progress  of  the  race 
can  be  directly  and  permanently  affected  by  improve- 
ments in  education  and  the  amelioration  of  social 
conditions.  It  is  certain  that  the  qualities  of  any 
person  — health,  character,  efficiency,  and  so  forth, 
depend  upon  his  environment  and  upbringing,  as  well 
as  upon  his  hereditary  endowments.  It  is  therefore 
necessary,  before  we  proceed  farther,  to  arrive  at  some 
estimate  of  the  relative  importance  of  inherent  qualities 
and  of  education — of  nature  and  nurture  respectively. 
Practically  the  only  piece  of  good  evidence  upon  this 
point  is  one  which  we  owe  to  the  researches  of  Galton. 

Galton’s  data  are  derived  from  the  history  of  twins. 
Now,  human  twins  are  of  two  kinds.  One  of  these 
kinds  corresponds  to  the  young  of  those  animals  which 
normally  bear  two  or  more  at  a birth,  each  being 
derived  from  a separate  ovum,  whilst  the  second  kind 
is  the  result  of  the  development  of  two  embryos  from 


288 


EUGENICS 


the  same  fertilized  ovum.  In  the  former  case  the 
twins  bear  no  more  resemblance  to  one  another  than 
any  other  ordinary  pair  of  brothers  or  sisters,  and  they 
are  often  of  opposite  sex  ; in  the  latter  case  the  twins 
are  known  as  ‘ identical,’  and  are  always  of  the  same 
sex.  Our  present  knowledge  of  Genetics — not,  of 
course,  available  to  Galton  when  he  first  wrote  upon 
this  subject — leads  us  to  believe  that  such  twins  are 
indeed  identical,  and  bear  precisely  the  same  hereditary 
endowment.*  It  is  as  though  a single  individual  were 
divided  into  two  parts,  and  each  part  grew  into  a 
complete  person.  Galton  quotes  numerous  stories 
of  the  frequent  confusion  between  identical  twins. 
‘ I have  one  case,’  he  writes,  ‘ in  which  a doubt  remains 
whether  the  children  were  not  changed  in  their  bath, 
and  the  presumed  A is  not  really  B,  and  vice  versa.’ 
In  the  records  thus  collected  there  is  excellent  material 
for  discovering  how  far  identical  twins  can  come  to 
differ  from  one  another  when  exposed  to  different 
conditions,  and,  on  the  other  hand,  for  ascertaining 
how  far  distinct  twins  brought  up  under  similar  con- 
ditions can  come  to  resemble  one  another.  Galton 
obtained  information  with  regard  to  eighty  cases  of 
probably  identical  twins.  In  many  of  these  cases  the 
twins  remained  closely  alike  in  temper  and  character, 
as  well  as  in  appearance,  up  to  an  advanced  age. 
When  differences  arose  in  later  life,  these  were  generally 
due  to  some  illness  or  accident  which  affected  one  of 
the  twins.  The  gradual  influence  of  a number  of  small 
causes  seemed  to  have  very  little  effect  in  this  respect. 

* [Cases  are,  however,  known  In  which  twins,  apparently 
‘ identical  ’ in  most  characters,  differ  in  one  or  more  which  are 
certairffy  congenital.] 


EFFECT  OF  ENVIRONMENT 


289 


‘ In  not  a single  instance,’  Galton  writes,  ‘ have  I met 
with  a word  about  the  growing  dissimilarity  being  due 
to  the  action  of  the  firm,  free  will  of  one  or  both  the 
twins  which  had  triumphed  over  natural  tendencies ; 
and  yet  a large  proportion  of  my  correspondents  happen 
to  be  clergymen,  whose  bent  of  mind  is  opposed,  as  I 
feel  assured,  to  a necessitarian  view  of  life.’  The  only 
conclusion  which  can  be  drawn  from  these  observations 
is  that  the  relative  influence  of  nurture  as  opposed  to 
nature  is  very  much  smaller  than  has  been  generally 
supposed;  and  this  inference  is  confirmed  by  the 
history  of  dissimilar  twins.  The  descriptions  of  the 
latter  agreed  without  exception  in  showing  an  entire 
absence  of  convergence  of  character  in  cases  where 
the  whole  training  and  environment  were  closely 
similar.  From  this  evidence  it  seems  right  to  conclude 
that  the  hereditary  nature  of  a man  is  more  important 
than  his  training  and  circumstances  in  determining 
his  adult  mental  and  physical  equipment,  and  the 
result  of  common  observation  may  be  said  to  be  in 
agreement  with  this  conclusion. 

You  may  educate  generation  after  generation,  and 
yet  the  starting-point  from  which  each  individual  has 
to  begin  his  struggle  upwards  may  remain  the  same, 
even  though  each  may  struggle  a little  farther  than  the 
one  who  came  before  him.  On  the  other  hand,  we 
have  all  of  us  met  a few  of  those  happy  people  to 
whom  it  seemed  second  nature  to  do  the  right  thing, 
and  for  whom  the  difficulties  of  life  appear  to  have 
no  menace.  These  qualities  are  those  of  nature,  and 
not  of  nurture,  and  their  children  will  inherit  them. 
Some  are  born  great,  some  achieve  greatness,  and 

19 


290 


EUGENICS 


some  have  greatness  thrust  upon  them.  But  those 
who  are  bom  great — meaning  by  this  not  those  in 
high  position,  but  great  in  themselves — are  the  men 
and  women  to  whose  descendants  we  must  look  for 
the  future  greatness  of  mankind. 

If  this  view  be  justified,  we  shall  be  obliged  to  revise 
very  carefully  our  ideas  of  what  is  desirable  in  social 
legislation.  Important  as  education,  sanitation,  and 
the  like  may  be,  their  effects  are  strictly  limited. 
;The  relative  birth-rate  of  good  and  bad  stock,  on  the 
other  hand,  is  the  fundamental  factor.  Its  influence 
in  a single  generation  may  be  so  small  as  to  be  barely 
recognizable,  but  its  effect  increases  from  generation 
to  generation,  and,  moreover,  it  is  an  effect  which,  once 
produced,  is  quite  irrevocable.  We  have  to  consider, 
therefore,  how  the  relative  incidence  of  the  birth-rate 
falls  at  the  present  time,  and  what  are  the  causes 
which  affect  it.  We  have  to  consider  especially 
whether  existing  and  proposed  legislation  of  which  the 
intention  is  to  improve  the  education  and  condition 
of  upbringing  of  children  has  any  effect,  direct  or  in- 
direct, upon  the  relative  birth-rate  of  different  classes 
of  society.  For  if  such  legislation  be  found  to  favour 
the  rapid  reproduction  of  the  less-efficient,  it  will 
become  a matter  for  serious  consideration  whether  the 
advantages  of  mental  and  physical  improvement  in  the 
individual  are  not  being  purchased  too  dearly  at  the 
expense  of  posterity. 

Individuals,  as  well  as  Governments,  will  do  well 
to  look  closely  into  the  possible  results  of  the  best- 
intentioned  proceedings.  To  quote  Galton  once  more : 


SIZE  OF  FAMILIES 


291 


* It  is  known  that  a considerable  part  of  the  huge 
stream  of  British  charity  furthers  by  indirect  and 
unsuspected  ways  the  production  of  the  Unfit ; it  is 
most  desirable  that  money  and  other  attention  be- 
stowed on  harmful  forms  of  charity  should  be  diverted 
to  the  production  and  well-being  of  the  Fit.  For 
clearness  of  explanation  we  may  divide  newly  married 
couples  into  three  classes,  with  respect  to  the  probable 
civic  worth  of  their  offspring.  There  would  be  a small 
class  of  “ desirables,”  a large  class  of  ” passables,”  of 
whom  nothing  more  will  be  said  here,  and  a small 
class  of  " undesirables.”  It  would  clearly  be  advan- 
tageous to  the  country  if  social  and  moral  support,  as 
well  as  timely  material  help,  were  extended  to  the 
" desirables,”  and  not  monopolized,  as  it  is  now  apt 
to  be,  by  the  “ undesirables.” 

Let  us  consider  the  relative  birth-rates  of  different 
classes  of  the  community.  This  is  in  itself  a very  large 
question.  We  have  only  space  here  to  record  a few 
figures,  taken  from  a paper  by  Prof.  Karl  Pearson 
than  whom  no  better  authority  could  be  quoted. 

Here  are  a few  of  the  figures,  in  terms  of  the  average 
size  of  family,  childless  marriages  being  excluded  ; 


Group  I. 


Criminals  . . 

English  deaf-mutes  . . 
London,  mentally  defective 


6-6 

62 

7-0 


Group  II. 


English  middle  class 
London  normal  artisan 
English  intellectual  class 


292 


EUGENICS 


This  means,  if  one  takes  into  consideration  the  fact 
that  members  of  the  first  group  probably  marry  several 
years  earlier,  on  the  average,  than  members  of  the 
second  group,  that  the  mentally  defective  class  is 
reproducing  itself  about  t\yice  as  rapidly  as  the  intel- 
lectual class.  There  can  be  no  doubt  as  to  what  the 
result  of  such  a proportion  as  this  must  be.  Let  us 
quote  Prof.  Pearson’s  own  conclusion  : ‘ The  progress 
of  the  race  inevitably  demands  a dominant  fertility 
in  the  fitter  stocks.  If  that  principle  be  not  recognized 
as  axiomatic  by  the  mentally  and  bodily  fit  themselves, 
if  the  statesman  do  not  accept  it  as  a guide  in  social 
legislation,  then  the  race  will  degenerate  until,  sinking 
into  barbarism,  it  may  rise  again  through  the  toilsome 
stages  of  purification  by  crude  natural  selection.’  Or, 
it  may  be  added,  until  it  be  annihilated  by  a more 
efficient  neighbour. 

With  regard  to  the  cause  of  these  differences  in  the 
birth-rate  of  different  sections  of  the  community,  there 
can  be  very  little  doubt  that  they  are  largely  due  to 
deliberate  restraint  on  the  part  of  the  more  prudent 
and  intelligent  classes.  When  a person  has  attained 
by  his  own  industry  to  a certain  standard  of  comfort, 
and  even  luxury,  he  naturally  desires  to  give  his 
children  a fair  start,  and  to  insure  for  them  an  equal 
degree  of  ease  and  security  ; and  if  he  finds  that  this  is 
more  easily  done  when  the  number  of  his  family  is 
limited,  it  is  difficult  to  blame  him  for  taking  advantage 
of  the  fact.  If  this  class  of  men  had  more  children 
on  the  average  than  are  produced  by  people  in  in- 
ferior stations,  it  would  mean  that  the  children  of  the 


CARE  OF  INFERIOR  STOCKS 


293 


comfortably  situated  would  have  to  be  contented  with 
positions  somewhat  inferior,  on  the  average,  to  those 
of  their  parents.  This  is  precisely  the  condition  of 
affairs  most  desirable  from  the  point  of  view  of  race- 
improvement  and  from  that  of  national  efficiency, 
since  any  given  position  would  thus  be  recruited  from  a 
better  and  not  from  a worse  class  than  the  one  which 
previously  occupied  it.  The  individual  may  be  par- 
doned if  this  is  not  what  he  desires  to  happen  in  the 
case  of  his  own  children. 

National  education  and  the  proposed  feeding  and 
care  of  the  children  of  inferior  stocks  at  the  cost  of  the 
State  are  measures  which  will  have  certain  definite 
effects  upon  the  relative  birth-rates  of  different  classes. 

It  is  proposed  to  do  all  this  at  the  expense  of  the 
fitter  stock,  which  is  thus  rendered  still  less  capable 
of  raising,  as  well  as  still  less  disposed  to  raise,  large 
families  of  healthy  children.  Such  measures  can  only 
be  justified  by  making  at  the  same  time  every  possible 
effort  to  correct  these  dangerous  differences  in  the 
incidence  of  the  birth-rate.  Legislation  in  these  two 
directions  ought  to  go  hand  in  hand.  Indeed,  the 
improvement  in  the  supply  of  children  ought  for  every 
reason  to  precede  the  improvement  in  the  care  and 
education  of  children  ; for  if  the  State  cares  for  the 
children,  it  has  a right  to  insist  that  the  supply  of 
children  shall  be  the  best  possible,  and  this  is  far  from 
being  the  case  at  present. 

Remedies  for  the  existing  condition  of  things  have 
been  proposed  by  would-be  philanthropists  from  Plato 
downwards.  But  against  aU  suggestions  for  running 


294 


EUGENICS 


the  human  race  on  the  principles  of  the  stud  farm  the 
objection  holds  good  which  was  put  forward  by  Huxley : 
‘ Who  is  competent  to  do  the  necessary  selecting  ? 
How  can  the  pigeons  be  their  own  Sir  John  Sebright  ?’ 
‘ The  points  of  a good  or  of  a bad  citizen,’  says  Huxley, 
* are  really  far  harder  to  discern  than  those  of  a puppy 
or  a short-horn  calf.  Many  do  not  show  themselves 
before  the  practical  difficulties  of  life  stimulate  man- 
hood to  full  exertion.  And  by  that  time  the  mischief 
is  done.  The  evil  stock,  if  it  be  one,  has  had  time  to 
multiply,  and  the  selection  is  nullified.’  And  there  is 
another  objection.  The  ruthlessness  necessary  for  the 
carr3fing  out  of  the  method  of  deliberate  selection  is 
in  itself  so  imsocial  a quality  that,  if  it  were  ever  to 
arise,  society  would  probably  be  far  worse  off  than 
before.  The  method  is  in  itself  directly  opposed  to 
the  development  of  the  higher  social  qualities. 

The  student  of  Eugenics  must  therefore  endeavour 
to  devise  other  methods,  both  for  encouraging  the 
fertility  of  the  better  stock  and  for  discouraging  that 
of  the  inferior  stock.  A considerable  number  of 
specific  suggestions  have  already  been  made.  ‘ Not 
a few  medical  men,’  writes  Heron,  ‘ are  urging  that 
propagation  among  the  obviously  unfit — those  affected 
with  definite  hereditary  taints  : the  imbeciles,  the 
idiotic,  the  sufferers  from  S3rphilis  and  tuberculosis, 
should  be  authoritatively  restrained.’  Can  it  be  urged 
that  such  a proceeding  would  be  unduly  tyrannous  ? 
Surely  if  these  people  understood  the  irrevocable  laws 
of  heredity — if  they  only  knew — they  would  be  them- 
selves unwilling  to  hand  on  a tainted  existence  to 


OBJECT  OF  EUGENICS 


295 


future  generations  ! If  there  are  people  so  debased 
that  this  argument  does  not  appeal  to  them,  surely 
such  a crime  against  Society  as  a marriage  of  this  kind  is 
at  least  as  open  to  coercive  treatment  as  many  of  the 
acts  which  are  treated  as  criminal  by  existing  laws  ! We 
elaborately  prevent  and  punish  paltry  offences  against 
property,  and  yet  deliberate  crimes  like  marriages 
between  the  Unfit  are  not  recognized  as  criminal. 

Various  suggestions  for  encouraging  the  multiplica- 
tion of  the  Fit  have  also  been  made. 

Mr.  Sidney  Webb,  in  his  ‘ Decline  of  the  Birth-Rate,' 
has  suggested  indiscriminate  help  to  all  parents,  since 
this  should  afford  encouragement  to  those  who  limit 
their  families  for  prudential  reasons  and  at  the  same 
time  leave  the  thriftless  where  they  are.  But  here 
it  is  necessary  to  point  out  that  the  existence  of  an 
unlimited  population  must  of  necessity  bring  want 
and  misery  to  the  lowest  strata  of  society.  The  first 
object  of  Eugenics,  Galton  tells  us,  ‘ is  to  check  the 
birth-rate  of  the  unfit,  instead  of  allowing  them  to 
come  into  being,  though  doomed  in  large  numbers  to 
perish  prematurely.  The  second  object  is  the  improve- 
ment of  the  race  by  furthering  the  productivity  of 
the  Fit  by  early  marriages  and  healthful  rearing  of 
their  children.  Natural  selection  rests  upon  excessive 
production  and  wholesale  destruction  ; Eugenics  upon 
bringing  into  the  world  no  more  individuals  than  can 
properly  be  cared  for,  and  those  only  of  the  best 
stock.' 

In  the  ideal  socialistic  community,  in  which,  in 
addition  to  all  the  present  varieties  of  civil  servants, 


3^6 


EUGENICS 


all  those  engaged  in  education,  all  members  of  the 
Legislature,  all  doctors  and  all  lawyers,  would  no  doubt 
derive  their  incomes  from  the  State,  a recent  sugges- 
tion of  Mr.  McDougal’s  might  be  put  into  practice. 
This  is  to  graduate  incomes  according  to  the  number 
of  children.  Thus  the  position  of  married  people 
would  be  made  much  fairer  on  some  such  scheme  as 
the  following : Supposing  the  salary  of  a particular 
post  under  existing  conditions  to  be  £700  a year,  a 
bachelor  occupying  the  post  would  be  paid  only  £400 
a year  (say).  On  his  marriage  an  addition  of  £100  per 
annum  would  be  made  to  his  income,  and  a similar 
increment  would  again  take  place  at  the  birth  of 
every  child.  Under  the  conditions  here  postulated 
this  system  would  apply  to  the  bulk  of  the  more 
intellectual  members  of  the  community,  and,  incident- 
ally, it  would  have  a special  advantage,  to  which  we 
may  now  make  allusion.* 

Although  a nation’s  welfare  depends  to  a very  great 
extent  upon  the  mental  and  bodily  health  of  the  rank 
and  file  of  its  citizens,  yet  the  birth  of  an  occasional 
genius  makes  an  enormous  difference  to  the  progress 
of  the  world.  Now,  Galton  has  shown  quite  con- 
clusively that  there  is  a much  greater  chance  of  a 
genius  appearing  among  the  children  of  eminently 
intelligent  parents  than  in  an  average  family.  There- 
fore, if  the  fertility  of  the  more  intellectual  classes  is 
encouraged,  the  chance  of  obtaining  a genius  now  and 
again  is  much  increased. 

* It  is  not  to  be  inferred  from  this  paragraph  that  the 
writer  is  an  advocate  of  socialism. 


CRIMINAL  TAINT  HEREDITARY  297 


It  must  not  be  supposed  that  the  writer  is  a special 
advocate  of  all  or  any  of  the  suggestions  which  have 
been  mentioned  above.  They  are  only  alluded  to  in 
order  to  indicate  the  directions  in  which  the  problem 
of  race- improvement  may  be  attacked. 

The  point  of  view  which  has  been  adopted  in  the 
present  chapter  is  very  well  summed  up  in  the  following 
paragraph  from  a paper  by  Prof.  Karl  Pearson  : 

‘ As  we  have  found  conscientiousness  is  inherited, 
so  I have  little  doubt  that  the  criminal  tendency 
descends  in  stocks.  To-day  we  feed  our  criminals  up, 
and  we  feed  up  the  insane ; we  let  both  out  of  the 
prison  or  the  asylum  “ reformed  ” or  " cured,”  as  the 
case  may  be,  only  after  a few  months  to  return  to 
State  supervision,  leaving  behind  them  the  germs  of 
a new  generation  of  deteriorants.  The  average  number 
of  crimes  due  to  the  convicts  in  His  Majesty's  prisons 
to-day  is  ten  apiece.  We  cannot  reform  the  criminal 
nor  cure  the  insane  from  the  standpoint  of  heredity ; 
the  taint  varies  not  with  their  moral  or  mental  conduct. 
These  are  the  products  of  the  somatic  cells  ; the  disease 
lies  deeper  in  their  germinal  constitution.  Education 
for  the  criminal,  fresh  air  for  the  tuberculous,  rest  and 
food  for  the  neurotic — these  are  excellent.  They  may 
bring  control,  sound  lungs,  and  sanity  to  the  individual; 
but  they  will  not  save  the  offspring  from  the  need  of 
like  treatment,  nor  from  the  danger  of  collapse,  when 
the  time  comes.  They  cannot  make  a nation  sound  in 
mind  and  body  ; they  merely  screen  degeneracy  behind 
a throng  of  arrested  degenerates.  Our  highly  developed 
human  sympathy  will  no  longer  allow  us  to  watch 


EUGENICS 


298 

the  State  purify  itself  by  aid  of  crude  natural  selection. 
We  see  pain  and  suffering  only  to  relieve  it,  without 
inquiry  as  to  the  moral  character  of  the  sufferer,  or  as 
to  his  national  or  racial  value.  And  this  is  right.  No 
man  is  responsible  for  his  own  being  ; and  nature  and 
nurture,  over  which  he  had  no  control,  have  made  him 
the  being  he  is,  good  or  evil.  But  here  science  steps 
in,  crying,  “ Let  the  reprieve  be  accepted,  but  next 
remind  the  social  conscience  of  its  duty  to  the  race. 
No  nation  can  preserve  its  efficiency  unless  dominant 
fertility  be  associated  with  the  mentally  and  physically 
fitter  stocks.  The  reprieve  is  granted,  but  let  there 
be  no  heritage  if  you  would  build  up  and  preserve  a 
virile  and  efficient  people.’ 

Signs  are  not  wanting  that  a few  thoughtful  people 
outside  the  ring  of  scientific  enthusiasts  are  beginning 
to  take  a serious  interest  in  the  questions  of  race- 
improvement  and  national  efficiency.  But  before  any 
real  progress  is  likely  to  be  made,  a knowledge  of  the 
fundamental  facts  of  heredity  must  have  become  very 
much  more  widely  distributed  than  is  at  present  the 
case.  Valuable  work  is  being  done  in  this  direction 
by  the  Eugenics  Education  Society,  the  object  of  which 
is  to  popularize  and  carry  into  practical  effect  the  idea 
of  national  Eugenics,  as  defined  at  the  beginning  of 
the  present  chapter.  The  intention  of  this  society  to 
make  known  generally  the  fact  that  biological  principles 
apply  to  the  production  of  human  beings  is  one  which 
cannot  be  too  highly  estimated.  Since  this  is  pre- 
cisely the  object  with  which  the  present  sketch  has  been 
added  to  a work  otherwise  almost  purely  theoretical 


BIBLIOGRAPHY 


299 


in  character,  I hare  thought  it  proper  to  conclude 
my  chapter  on  Eugenics  with  a brief  reference  to  the 
aims  and  objects  of  this  society. 


Galton,  F.  : 

$t 

BIBLIOGRAPHY. 

: Hereditary  Genius.  1869. 

English  Men  of  Science.  1874. 

Inquiries  into  Human  Faculty  and  its  Develop- 
ment. 1883. 

9» 

97 

Natural  Inheritance.  1889. 

Eugenics  : Its  Definition,  Scope,  and  Aimfc 

(Sociological  Society  Papers,  1905). 
The  Eugenics  Review,  1909 — 

Schuster,  E.:  Eugenics,  1912.  (Collins.) 


CHAPTER  XI 


CONCLUDING  CHAPTER 

In  the  preceding  chapters  a considerable  variety  ol 
topics  has  been  dealt  with,  and  in  spite  of  the  fact  that 
all  are  more  or  less  intimately  connected  with  the 
study  of  organic  evolution,  the  nearly  historical  order  in 
which  the  subject-matter  has  been  in  great  part  pre- 
sented has  inevitably  rendered  the  treatment  a little 
disjointed. 

The  method  we  have  so  far  adopted  serves  to  illus- 
trate the  state  of  transition  in  which  our  studies 
stand,  and  which  it  is  our  first  object  to  assist  in 
hastening  to  a close — the  transition  between  the  specu- 
lative philosophy  of  evolution  and  the  exact  science  of 
genetics. 

Future  treatises  on  genetics  will  make  a fair  beginning 
with  the  law  of  Mendel,  and  will  then  deal  with  the 
application  of  this  law  in  detail ; and  in  this  concluding 
chapter  we  may  adopt  the  same  method,  and  proceed 
to  show  how  Mendel’s  discovery  affords  the  connecting- 
hnk  between  the  various  divergent  branches  which  we 
have  already  sketched  in  outline. 

The  central  generalization,  then,  around  which  the 
subjects  considered  in  the  preceding  pages  are  foimd 

300 


THE  SCIENCE  OF  GENETICS 


301 


naturally  to  group  themselves  is  afforded  by  the  law 
of  inheritance  discovered  by  the  Abbe  Mendel  about 
the  year  1865.  This  discovery  has  rendered  possible 
that  rapid  advance  of  the  science  of  genetics,  or  the 
study  of  the  hereditary  phenomena  of  organisms, 
which  has  taken  place  during  the  first  few  years  of  the 
twentieth  century.  It  is  the  writer’s  avowed  opinions 
that  Mendel’s  brief  paper  is  the  most  important  con- 
tribution of  its  size  which  has  ever  been  made  to  bio- 
logical science.  Little  apology  is  therefore  needed  for 
formulating  once  again  the  law  based  by  Professor 
Correns  upon  the  conclusions  which  this  paper  contains. 

Mendel’s  law  relates  to  the  inheritance  of  certain 
definite  characters,  which  have  since  been  called  allelo- 
morphs. It  is  a distinctive  feature  of  allelomorphic 
characters  that  they  are  found  to  group  themselves 
naturally  into  pairs  of  more  or  less  antagonistic 
qualities.  In  many  cases  the  pair  is  represented  by 
the  presence  and  absence  respectively  of  a certain 
definite  feature.  The  two  allelomorphs  of  a pair  may 
be  conveniently  written  as  A and  a. 

We  have  seen  that  the  cells  of  zygotic  organisms — 
organisms,  that  is  to  say,  which  have  arisen  by  the 
process  of  sexual  reproduction — contain  a double 
complement  of  hereditary  qualities.  Such  cells  may 
contain  A and  A,  a and  a,  or  A and  a.  The  forms  A A 
and  aa  are  described  as  homozygotes,  the  form  A a as 
a heterozygote.  In  the  simpler  cases  we  are  enabled 
to  study  the  behaviour  of  such  a single  pair  of  allelo- 
morphs by  itself,  without  reference  to  any  other  features 
which  the  animals  or  plants  under  consideration  may 


302 


CONCLUDING  CHAPTER 


display.  The  demonstration  that  there  exist  definite 
and  separable  unit  characters  of  this  kind  is  the  first 
great  debt  that  science  owes  to  Mendel. 

Up  to  the  present  our  certain  knowledge  of  the 
Mendelian  behaviour  of  unit  characters  has  been  con- 
fined to  cases  of  cross-breeding.  In  the  simplest  case 
which  we  have  to  consider,  two  homozygote  forms,  A A 
and  aa,  are  crossed  together. 

The  external  character  or  visible  appearance  of  the 
heterozygote  A a,  produced  in  this  manner,  differs  in 
different  cases.  In  the  commonest  case  A represents 
the  dominant  allelomorph,  and  in  this  case  the  appear- 
ance of  the  heterozygote  is  practically  indistinguish- 
able from  that  of  the  homozygote  A A.  In  other  cases 
the  heterozygote  Aa  is  different  in  appearance  from 
either  homozygote  AA  or  aa.  Sometimes  Aa  is  inter- 
mediate between  AA  and  aa,  in  other  cases  it  is  to  all 
appearances  totally  distinct  from  either. 

So  much  for  the  external  appearance  of  homozygote 
and  heterozygote  forms.  In  the  production  of  the 
gametes,  or  germ-cells,  we  arrive  once  more  at  the 
simplest  possible  form  of  hereditary  constitution,  foi 
we  beheve  each  feature  in  the  body  to  be  represented 
in  the  germ-cells  by  a single  determining  factor  only. 
Still  confining  our  attention  to  the  representatives  of 
a single  pair  of  allelomorphs,  we  find  that  aU  the  germ- 
cells  of  a homozygote  contain  only  A or  only  a , as  the  case 
may  be.  But  in  the  case  of  the  germ-ceUs  derived  from 
a heterozygote,  A and  a are  represented  in  an  equal 
number  of  the  gametes  produced  by  the  same  individual. 
And  the  separation  between  the  two  allelomorphs  is 
found  in  almost  all  cases  to  be  perfectly  complete. 


MENDEL’S  LAW 


303 


This  complete  segregation  of  the  two  allelomorphs  in 
equal  numbers  of  the  germ-cells  of  a heterozygote 
constitutes  the  first  and  most  important  section  of 
the  generalization  known  as  Mendel’s  law. 

The  second  part  of  the  law  refers  to  the  fact  that, 
as  a general  rule,  separate  pairs  of  allelomorphs  segre- 
gate quite  independently  of  one  another.  To  this  rule 
a few  exceptions  have  been  recorded  in  cases  where 
apparently  distinct  pairs  of  determining  factors  behave 
in  segregation  like  a single  pair  of  allelomorphs.  In 
such  cases  we  regard  the  members  of  the  distinct  pairs 
of  allelomorphs  as  being  coupled  together,  although  no 
serious  attempt  has  yet  been  made  to  picture  the  way 
in  which  this  coupling  comes  about.*  In  other  cases  the 
coupling  is  only  partial ; in  others,  again,  there  is  repul- 
sion between  allelomorphs  belonging  to  distinct  pairs. 

The  fact  that  in  the  great  majority  of  cases  separate 
pairs  of  allelomorphs  segregate  independently  of  one 
.another  leads  to  the  possibility  of  new  combinations  of 
the  parental  characters  being  formed  in  the  germ-cells 
of  the  cross-bred  individuals  ; in  fact,  this  must  alwa}^ 
happen  when  the  parent  types  differ  in  more  than 
one  pair  of  segregable  characters.  When  two  similar 
germ-cells,  each  bearing  the  same  new  combination  of 
allelomorphs,  meet  together  in  fertilization,  the  result 
is  a new  zygotic  combination  which  is  a pure  type  in 
respect  of  the  characters  concerned,  and  henceforth 
breeds  true.  Thus  if  AB  . AB  is  crossed  with  ab  . ab 

* [Prof.  T.  H.  Morgan  and  his  associates  have  brought  forward 
remarkable  evidence,  from  their  work  on  the  Fruit-fly  (Droso- 
phila), that  this  coupling  is  due  to  the  determining  factors  of  the 
coupled  allelomorphs  being  borne  by  the  same  chromosome. 
See  pp.  269-271.] 


/ 


304  CONCLUDING  CHAPTER 

the  heterozygote  AB  . ab  produces  in  equal  numbers 
the  genn-ceUs  AB,  Ab,aB,  and  ab.  Among  the  combina- 
tions of  these  germ-cells  which  are  represented  by  the 
various  offspring  of  the  heterozygote  there  must  appear 
Ab  . Ab  and  aB  . aB — novel  types  which  are  pure  in 
constitution,  and  which  may  form  the  starting-points 
for  new  strains  or  races. 

Upon  this  fact  depends  the  enormous  importance  of 
Mendel’s  law  in  the  breeding  of  new  and  useful  types 
of  animals  and  plants.  When  it  is  remembered  that  in 
wheat,  for  example,  resistance  and  non-resistance  to 
the  attacks  of  disease,  earliness  and  lateness  of 
ripening,  good  and  bad  milling  quality,  are  aU  pairs  of 
Mendelian  allelomorphs,  and  that  it  is  now  possible  to 
take  a different  example  of  these  qualities  from  each 
of  three  different  strains,  and  to  combine  them  together 
in  a single  new  variety  with  perfect  certainty  and  in 
four  generations,  it  does  not  require  much  imagination 
to  foresee  that  every  department  of  the  animal  and 
plant  breeding  industries  must  sooner  or  later  benefit 
enormously  from  Mendel’s  discovery. 

So  far  we  have  only  been  dealing  with  the  very  simplest 
of  Mendelian  phenomena,  leading  to  the  arithmetical 
addition  and  subtraction  of  definite  visible  characters. 
Other  kinds  of  allelomorphs  also  exist  which  undergo 
a similar  process  of  segregation  during  gamete  forma- 
tion, following  Mendel’s  law  in  a perfect  manner ; but 
which  may  remain  entirely  invisible  and  unsuspected 
so  long  as  certain  other  allelomorphs,  belonging  to 
quite  distinct  pairs,  are  excluded  from  the  zygotes  in 
which  these  invisible  factors  are  concealed.  When 


MENDEL’S  LAW 


305 


this  other  complementary  allelomorph  is  introduced; 
however,  by  crossing  with  an  individual  which  contains 
it,  the  feature  previously  hidden  becomes  visible,  giving 
rise  to  the  phenomenon  which  has  long  been  familiar 
under  the  name  of  reversion  on  crossing.  The  demon- 
stration of  these  invisible  factors,  and  of  the  fact  that 
tltey  also  obey  Mendel’s  law  with  perfect  regularity, 
is  surely  one  of  the  most  remarkable  discoveries  which 
have  ever  been  made  in  the  whole  history  of  biology. 
This,  again,  is  a piece  of  knowledge  which  may  be  of 
the  very  greatest  importance,  not  only  to  breeders  of 
bright  flowers,  some  of  which  are  already  known  to 
exhibit  the  phenomenon  described,  but  also  in  all 
classes  of  breeding  work  where  similar  facts  doubtless 
await  discovery. 

To  the  man  of  science,  however,  the  practical  aspect 
of  these  achievements  wiU  be  of  little  account  in  com- 
parison with  the  importance  of  their  application  to  the 
advance  of  human  knowledge  in  that  most  fascinating 
of  scientific  studies — biology.  Let  us,  then,  turn  to 
consider  the  way  in  which  Mendel’s  discovery  affects 
other  branches  of  biological  science. 

We  have  so  recently  had  occasion  to  point  to  the 
remarkable  coalition  between  Mendelism  and  cytology 
that  little  more  need  be  said  here  upon  the  subject. 
Mendel’s  theory  has,  indeed,  thrown  a flood  of  light 
upon  the  meaning  of  the  microscopic  phenomena 
exhibited  by  the  minute  constituent  parts  of  the  cells 
of  living  organisms,  phenomena  the  meaning  of  which 
could  only  be  vaguely  guessed  at  previously  to  the 
introduction  of  the  new  method. 


20 


30b 


CONCLUDING  CHAPTER 


The  intimate  connection  between  Mendelism  and 
C5d;ology  rests  to  a large  extent  upon  the  close  parallel 
which  exists  between  the  behaviour  of  allelomorphic 
characters  on  the  one  hand  and  that  of  chromosomes 
on  the  other. 

In  the  germ-cells  of  the  higher  animals  the  allelo- 
morphs of  the  Mendelian  become  segregated,  being 
reunited  in  fertilization,  and,  as  a consequence,  the 
cells  of  the  zygote  contain  twice  as  many  of  these 
factors  as  do  the  gametes  or  germ-cells  themselves. 

Similarly,  in  the  cell  processes  upon  which  the  vital 
functions  of  the  higher  animals  are  founded,  the 
number  of  chromosomes  characteristic  of  somatic  or 
zygotic  cells  becomes  halved  at  the  formation  of  the 
gametes,  the  double  number  being  restored  by  the 
association  of  chromosomes  derived  from  two  separate 
gametes  in  the  process  of  fertilization.  We  have  said 
that  in  the  higher  animals  the  gametes  are  sometimes 
spoken  of  as  constituting  an  ‘ x-  generation,  which 
alternates  with  the  ‘ 2X-  generation  represented  by 
the  zygote.  We  may  justify  the  use  of  these  expressions 
by  a brief  comparative  statement  of  the  facts  relating 
to  the  two  so-called  generations  which  recur  in  the 
hfe-history  of  certain  families  of  plants.  In  doing  so 
we  shall  begin  our  account  with  the  most  primitive 
and  simplest  forms,  and  then  pass  on  to  other  types 
which  are  regarded  as  standing  on  higher  planes  of 
evolution. 

What  are  probably  some  of  the  most  primitive 
members  of  the  vegetable  kingdom  belong  to  the 
class  of  the  green  algae.  This  group  includes  a great 


CYTOLOGY  OF  PLANTS 


307 


number  of  comparatively  lowly  organisms,  the  majority 
of  which  dwell  submerged  beneath  the  surface  of  fresh 
or  salt  water.  In  such  members  of  the  green  algae  as 
have  so  far  been  examined  from  this  point  of  view,  it 
would  appear  that  the  2;v-generation  is  exclusively 
represented  by  the  single  cell  which  arises  as  the 
actual  product  of  conjugation  between  a pair  of 
gametes.  Reduction  takes  place  in  the  actual  zygotic 
ceU,  so  that  each  of  the  products  of  this  cell’s  division 
shows  once  more  the  reduced  number  of  chromosomes. 
Thus  the  great  bulk — the  vegetative  mass — of  the 
species  is  constituted  by  the  j;-generation,  and  the 
2A;-generation  is  composed  of  a single  ceU  only — a state 
of  things  which  is  exactly  the  reverse  of  what  is  to  be 
seen  in  the  higher  animals. 

In  the  vegetable  kingdom  evolution  seems  to  have 
been  accompanied  by  a gradual  increase  of  the  2X- 
generation,  and  a corresponding  reduction  of  the 
A;-generation  in  point  of  importance.  Between  the 
two  extremes  afforded  by  the  algse  on  the  one  hand, 
and  the  flowering  plants  on  the  other,  we  can  trace  a 
series  of  intermediate  stages  represented  by  types  in 
which  many  other  features  also  must  be  regarded  as 
standing  on  intermediate  planes  of  organization. 

As  an  example  of  an  intermediate  condition  of  this 
kind,  we  may  take  the  case  of  the  ferns. 

The  fern  plant,  as  commonly  understood,  represents 
the  2A;-generation.  The  method  by  which  the  life- 
history  of  the  fern  plant  is  continued  is  by  the  forma- 
tion of  uniceUular  reproductive  bodies  which  are 

known  as  spores.  The  formation  of  the  spores  takes 

20—2 


3o8 


CONCLUDING  CHAPTER 


place  in  sets  of  four,  and  their  production  is  preceded  by 
a reducing  division,  so  that  each  spore  nucleus  contains 
half  as  many  chromosomes  as  the  nuclei  of  the  fem- 
plant — ^the  spores,  in  fact,  represent  the  initiation  of 
the  X generation. 

Spores  take  no  part  in  any  process  of  conjugation. 
They  at  once  germinate  and  enter  on  an  embryonic 
development  of  their  own,  giving  rise  to  a considerable 
mass  of  cells,  all  of  which  contain  the  reduced  number 
of  chromosomes.  Thus  in  the  case  of  the  fern  we  have 
a small  but  well-developed  A;-generation  alternating 
with  a much  larger  2A?-generation.  The  mass  of 
cellular  tissue  making  up  the  Af-generation  has  been 
named  the  prothcdlus. 

Certain  cells  of  the  prothallus  develop,  without 
change  in  the  number  of  their  chromosomes,  into  the 
gametes.  These  are  differentiated  in  the  usual  way 
into  male  and  female — ova  and  spermatozoids  respec- 
tively. 

Fertihzation  of  the  ovum  by  the  spermatozoid  gives 
rise  to  a zygote  in  which  the  double  number  of  chromo- 
somes is  restored.  In  this  way  the  2:»:-generation  or 
fern  plant  is  initiated,  and  by  the  usual  processes  of 
cell  multiplication  and  differentiation  this  body 
becomes  completed,  developing  its  characteristic  fronds 
and  so  forth.  Thus  m the  ferns  the  2A?-generation  has 
arrived  at  a high  degree  of  development,  and  represents 
the  chief  bulk  of  the  plant.  The  a:-generation,  however, 
still  embodies  a considerable  mass  of  cells. 

Turning  to  the  higher  plants,  among  which  we  may 
include  those  which  produce  t5^ical  flowers  with 


CYTOLOGY  OF  PLANTS 


309 


stamens  or  with  pistils,  or  more  usually  with  both,  we 
find  that  the  A;-generation  has  become  still  further 
reduced,  so  that  it  no  longer  occupies  an  independent 
phase  of  the  life-history,  but  has  come  to  be  entirely 
dependent  upon  the  2»-generation  for  its  support. 

A plant  which  bears  both  stamens  and  pistils  gives 
rise  to  spores  of  two  kinds,  differing  greatly  in  size. 
The  smaller  spores  are  represented  by  the  pollen-grains, 
and  in  these,  after  one  or  two  cell  divisions,  unaccom- 
panied by  growth,  the  one  or  two  male  gametes  are 
produced.  The  small  association  of  cells  arising  in  this 
way  is  all  that  is  left  of  the  a;-generation  on  the  male 
side. 

The  nucleus  of  the  larger  spore  also  divides  a few 
times,  and  one  of  the  final  products  of  division  becomes 
the  ovum.  Spore  and  ovum,  as  well  as  the  few  inter- 
vening cells,  bear  the  reduced  number  of  chromosomes. 
The  a;-generation  thus  represented  is  never  set  free, 
but  remains  enclosed  in  the  tissues  of  the  a^f-generation 
right  up  to  the  time  of  fertilization.  In  the  process 
of  fertilization  the  double  number  of  chromosomes 
characteristic  of  the  2A:-generation  is  once  more 
arrived  at. 

We  can  look  upon  the  2A;-generation  of  the  higher 
plants  as  being  formed  by  an  expansion  of  the  fertilized 
ovum.  The  zygote,  instead  of  comprising  a single  ceU 
only,  by  dint  of  delaying  the  reducing  division,  has 
come  to  consist  of  a great  mass  of  cells,  all  the  nuclei 
of  which  contain  the  double  number  of  chromosomes. 
This  fact  is  also  our  excuse  for  applying  the  same  term 
of  zygote  to  the  ceU  produced  by  the  conjugation  of 


310 


CONCLUDING  CHAPTER 


gametes,  as  well  as  to  the  mass  of  cells  to  which  the 
zygote  (in  the  strictest  sense)  eventually  gives  rise. 
In  the  simplest  forms,  such  as  the  green  algae,  the  cell- 
and  nuclear-fusion  constituting  conjugation  are  imme- 
diately followed  by  fusion  of  the  chromosomes,  an 
event  which  we  have  seen  to  be  the  first  step  towards 
a reduction  in  the  number  of  these  bodies.  In  the 
higher  plants,  by  delaying  this  fusion  of  chromosomes 
until  many  cell  generations  later  than  the  fusion  of  the 
nuclei,  the  advantages  associated  with  the  possession 
of  a double  nucleus  have  been  obtained  for  a large  and 
complicated  mass  of  cells.  And  this  mass  has  gradu- 
ally advanced  in  organization  and  relative  importance, 
until  ultimately  the  ^-generation  has  been  reduced 
almost  to  the  vanishing  point. 

The  sex-phenomena  of  the  higher  animals  can  most 
readily  be  brought  into  line  with  those  of  the  higher 
plants  if  we  consider  that  in  animals  the  spore  and  the 
gamete  are  identical ; the  x-generation  is  here  con- 
densed into  the  smallest  possible  limits — namely,  those 
of  a single  cell. 

A female  animal  produces  ova,  and  a male  produces 
spermatozoa.  Similarly,  we  may  regard  as  a female 
plant  one  which  produces  only  the  larger  variety  of 
spores  from  which  ova  arise  ; and  we  may  regard  as  a 
male  plant  one  which  produces  only  pollen.  It  is 
much  more  usual  to  find  a flowering  plant  bearing  both 
pistils  and  stamens,  and  producing  both  large  and  small 
spores.  Such  an  organism  is  described  as  herma- 
phrodite^bearing  both  sexes.  Among  animals  ex- 
amples of  hermaphrodite  species  are  also  not  infrequent, 


CYTOLOGY  OF  PLANTS 


311 

and  here,  just  as  in  the  case  of  plants,  whole  families 
may  display  this  method  of  reproduction. 

We  see,  then,  that  the  course  of  evolution  in  the 
vegetable  kingdom  would  appear  to  have  been  accom- 
panied by  a gradual  increase  in  the  2X-generation 
at  the  expense  of  the  ^-generation.  Starting  with 
lowly  aquatic  organisms,  and  passing  upwards  through 
the  mosses  and  ferns  to  the  flowering  plants,  we  find 
a steady  diminution  in  the  ^-generation,  whilst  the 
vegetative  labour  of  the  plant  is  taken  over  by  the 
2%-generation.  It  is,  therefore,  proper  to  suppose  that 
organisms  in  which  the  main  stage  in  the  hfe-history 
is  of  double  origin,  and  bears  a double  complement  of 
hereditary  factors,  have  some  advantage  over  organisms 
in  which  this  is  not  the  case.  We  cannot,  of  course,  be 
certain  as  to  the  exact  nature  of  this  advantage,  but 
we  may  point  out  that  it  is  only  in  the  former  kind 
of  organisms  that  the  operation  of  Mendel’s  law  can 
lead  to  the  production  of  new  combinations  of  parental 
characters  in  the  body  which  represents  the  main  stage 
of  the  life-history  ; and  that  this  circumstance  may 
possibly  lead  to  a greater  power  of  adaptability  to 
external  circumstances. 

Perhaps  the  most  interesting  application  of  the  infor- 
mation afforded  by  Mendel’s  discovery  is  shown  in  its 
bearing  upon  the  question  of  discontinuity  in  the  origin 
of  species.  The  fact  of  the  definite  and  discontinuous 
inheritance  of  the  differentiating  features  which  dis- 
tinguish cultivated  varieties  from  one  another  would 
point  very  plainly  to  a belief  that  such  differences  had 
arisen  in  a definite  and  discontinuous  manner,  even  if 


312 


CONCLUDING  CHAPTER 


we  did  not  actually  know  from  direct  evidence  that 
the  origin  of  new  races  under  cultivation  is  usually 
sudden  and  complete. 

It  is  not  necessary  to  repeat  Darwin’s  demonstration 
of  the  close  analogy  between  the  origin  of  varieties 
under  cultivation  and  the  origin  of  species  in  Nature. 
It  is  more  to  the  purpose  to  point  out  that  Mendel’s 
law  has  already  been  shown  to  hold  good  in  the  case 
of  many  differences  which  have  certainly  not  arisen 
under  cultivation,  and  that  we  have,  moreover,  sure 
knowledge  of  the  definite  and  spontaneous  origin  of 
some  natural  species. 

Here  we  arrive  at  a point  at  which  the  evidence  is 
not  yet  by  any  means  complete.  We  do  not  laiow 
whether  aU  or  even  many  specific  differences  obey 
Mendel’s  law  on  crossing,  and  a sharp  limit  is  put  to 
our  researches  in  this  direction  by  the  fact  that  so^many 
natural  hybrids  are  sterile.  StiU  less  do  we  know 
from  direct  evidence  whether  the  majority  of  natural 
species  have  arisen  discontinuously,  although  there  is 
much  circumstantial  evidence  which  points  to  the  con- 
clusion that  this  must  have  been  the  case. 

Clearly  this  discontinuous  method  of  variation  is 
likely  to  repay  some  further  discussion.  That  such 
mutation,  or  definite  variation,  is  a phenomenon  of 
the  germ-ceUs  follows  from  the  fact  that  every  germ- 
cell normally  bears  the  complete  specific  character. 
Bateson  has  shown  that  we  must  regard  mutation  as 
consisting  in  the  production  of  new  kinds  of  gametes, 
which  differ  from  those  normally  characteristic  of  the 
species.  Such  a change  is  most  readily  pictured  by 


DISCONTINUOUS  VARIATION 


313 


imagining  an  asymmetrical  nuclear  division  taking 
place  immediately  before  the  formation  of  the  germ- 
cells,  and  this  would  lead  us  to  expect  a mutating 
species  to  give  rise  to  more  than  one  new  kind  of 
offspring  at  the  same  time.  Such  was  actually  the  case 
with  the  (Enothera  Lamarckiana  studied  by  de  Vries ; 
and  this  observation  stands  as  the  most  complete  piece 
of  evidence  of  a mutating  species  so  far  known  to  us. 
We  may  be  assured,  then,  that  the  complete  potential 
nature  of  new  types  as  well  as  of  old  ones  is  already 
laid  down  in  the  germ-ceUs  previous  to  fertUization. 
As  Bateson  puts  it  : ‘ For  the  first  time  in  the  history  of 
evolutionary  thought  Mendel’s  discovery  enables  us  to 
form  some  picture  of  the  process  which  results  in 
genetic  variation.  It  is  simply  the  segregation  of  a 
new  kind  of  gamete,  bearing  one  or  more  characters 
distinct  from  those  of  the  type.  We  can  answer  one 
of  the  oldest  questions  in  philosophy.  In  terms  of 
the  ancient  riddle,  we  may  reply  that  the  owl’s  egg 
existed  before  the  owl ; or,  if  we  hesitate  about  the 
owl,  we  may  be  sure  about  the  bantam.’* 

Let  us  consider  a little  more  closely  the  evidence  of 
mutation  afforded  by  de  Vries’  studies  of  (Enothera 
Lamarckiana.  Semi-wild  specimens  of  this  species, 
when  transplanted  and  carefully  observed,  were  found 
to  yield  nearly  3 per  cent,  of  seedlings  which  differed 
definitely  from  their  parent,  and  among  these  mutants 
some  fifteen  distinct  new  sorts  were  described.  Some 
of  the  new  species  equalled  or  even  surpassed  the  parent 

* British  Association,  Cambridge,  1904.  Address  to  the 
Zoological  Section,  p.  14. 


314 


CONCLUDING  CHAPTER 


0.  Lamarckiana  in  vigour  and  prolific  habit,  and  two 
of  them  actually  became  established  side  by  side  with 
the  parent  type  without  man’s  assistance. 

It  is  unfortunate  from  the  point  of  view  of  de  Vries’ 
interpretation  of  this  case  that  the  behaviour  of 
0.  Lamarckiana  should  suggest  in  some  respects,  as 
Bateson  has  pointed  out,  the  phenomena  of  hybridiza- 
tion. It  must  be  observed  in  support  of  de  Vries’ 
view  that  the  species  appears  to  exhibit  the  same 
phenomenon  in  other  locahties,  and,  further,  that  it 
has  not  been  possible  to  make  any  suggestion  as  to  the 
second  species  with  which  the  pure  Lamarckiana  might 
be  supposed  to  have  been  crossed. 

From  one  point  of  view,  as  de  Vries  has  himself 
pointed  out,  mutation  in  (Enothera  is  clearly  a 
phenomenon  of  hybrids,  and  this  circumstance  of 
itself  introduces  considerable  complications  into  the 
story. 

We  saw  just  now  that  there  is  every  reason  for  the 
conviction  that  mutation  takes  place  in  the  germ- 
cells,  and  not  in  the  zygote  after  fertilization.  Since 
the  number  of  mutants  given  off  under  the  most 
favourable  circumstances  did  not  exceed  3 per  cent, 
of  the  total  offspring,  the  enormous  majority  of  mutated 
germ-cells  (on  de  Vries’  view)  must  unite  with  germ- 
ceUs  bearing  the  ordinary  specific  character.  Conse- 
quently, the  new  types  which  appear  will  in  most  cases 
have  originated  in  the  form  of  a cross  between  a mutated 
germ-cell  and  an  ordinary  germ-cell.  And  since  this 
is  not  the  final  limit  to  the  possible  complications  of  the 
case,  we  can  easily  recognise  that  the  complete  inter- 


THE  MUTATION  THEORY  315 

pretation  of  the  behaviour  of  (Enothera  Lamarckiana  is 
not  by  any  means  an  easy  matter. 

As  enunciated  by  de  Vries,  the  theory  of  mutation 
amounts  to  a very  complete  and  definite  hypothesis. 
A large  part  of  this  author’s  suggestions  are,  however, 
almost  purely  speculative,  and  for  this  reason  we  have 
treated  the  whole  at  somewhat  less  length  than  it  per- 
haps deserves.  Some  of  de  Vries’  speculations  are, 
indeed,  more  picturesque  than  convincing. 

Thus,  de  Vries  regards  the  number  of  unit  characters 
— each  of  which  has  arisen  by  a single  mutation — to 
be  quite  limited,  even  in  the  highest  organisms. 
Three  or  four  thousand  such  characters,  he  thinks, 
may  go  to  build  up  the  hereditary  endowment  of 
the  most  complicated  species.  He  further  supposes 
a period  of  mutation  to  recur  about  once  in  4,000 
years.  Four  thousand  multiplied  by  4,000  gives 
16,000,000 — the  number  of  years  required  to  evolve 
the  lords  of  creation  from  a ‘ primordial  protoplasmic 
atomic  globule.’  And  he  points  out  that  this  estimate 
is  well  within  the  limits  of  geological  time  as  allowed 
by  the  physicist.  In  this  way  de  Vries  believes  that 
his  mutation  theory  removes  a difficulty  which  besets 
the  selection  hypothesis — the  difficulty,  namely,  of 
insufficient  time.  The  selectionist  may  reasonably 
reply  that  the  amount  of  change  necessary  to  produce 
in  4,000  years,  by  the  gradual  method,  a difference 
equal  to  that  represented  by  a single  unit  character, 
might  very  well  be  quite  imperceptible  in  a single 
generation. 

We  may  summarize  our  present  conclusions  as  to 


3i6 


CONCLUDING  CHAPTER 


the  discontinuous  nature  of  species  in  the  following 
manner : A great  number  of  specific  characters  are, 
without  doubt,  definite  ; they  are  inherited  as  definite 
entities,  and  there  can  be  no  question  that  their  first 
coming  into  existence  was  a definite  event.  Every 
year  tends  to  increase  the  range  of  characters  to  which 
the  conception  of  discontinuity  has  to  be  applied, 
i Certain  groups  of  characters  do,  however,  seem  to 
exhibit  the  phenomena  of  continuity.  Let  us  endeavour 
to  arrive  at  some  closer  idea  as  to  the  nature  of  these 
characters. 

A study  of  continuous  variations  very  quickly  leads 
to  the  conclusion  that  the  variable  features  are  those 
which  are  especially  liable  to  modification  during  the 
lifetime  of  the  individual,  owing  to  the  action  of  ex- 
ternal circumstances.  Such  quantitative  features  of 
size  and  shape  and  number  of  parts  are  particularly 
plastic  in  the  case  of  plants. 

The  habit,  or  general  form  and  appearance,  of  a 
plant  is  a feature  very  characteristic  of  individual 
species.  The  presence  of  a dwarf  or  of  a tall  habit 
does,  indeed,  constitute  a frequent  distinction  between 
different  strains  of  garden  plants,  and  the  inheritance 
of  these  characters  in  many  cases  follows  Mendel’s 
law.  But  leaving  aside  this  particular  example,  the 
inheritance  of  habit  is  very  little  understood  ; although 
habit  is  a feature  which  is  very  liable  to  considerable 
fluctuations.  Habit  seems,  in  fact,  usually  to  afford 
an  example  of  continuous  variability. 

The  habit  of  some  species  of  plants  when  grown 
under  alpine  conditions  on  mountain  summits  is  so 


CONTINUOUS  VARIATIONS 


317 


different  from  that  of  the  same  species  when  growing 
in  the  plains,  that  inexperienced  persons  might  readily 
suppose  two  such  forms  to  belong  to  as  many  distinct 
species.  At  intermediate  levels  the  habit  is  more  or 
less  intermediate.  Bonnier  made  the  experiment  of 
dividing  individual  plants  into  two  portions,  plant- 
ing one  part  at  a high  elevation  and  the  other  near  the 
level  of  the  sea.  In  a few  years  the  plant  grown  on 
the  mountain  had  taken  on  the  full  alpine  habit, 
whilst  that  grown  on  the  plain  retained  the  ordinary 
appearance  of  the  species.  In  this  way  very  con- 
siderable differences  in  habit  were  shown  to  be  directly 
dependent  on  external  conditions. 

In  some  few  cases  the  environment  determines  the 
production  of  perfectly  definite  and  discontinuous 
features.  The  water  ranunculus,  when  growing  sub- 
merged beneath  the  surface  of  a pond,  produces  leaves 
the  blades  of  which  are  cut  up  into  a great  number  of 
fine  thread-like  segments.  As  soon  as  the  top  of  the 
plant  reaches  the  surface  of  the  water  those  leaf  rudi- 
ments which  are  just  commencing  their  existence  pro- 
ceed to  develop  in  a totally  different  fashion.  The 
leaves  to  which  they  give  rise  possess  a wide  and  undi- 
vided blade,  which  floats  upon  the  surface  of  the  water. 
The  two  sorts  of  leaves  are  as  utterly  different  in 
appearance  as  it  is  possible  for  leaves  to  be.  Yet  the 
effect  of  external  conditions  upon  the  yoimg  leaf- 
rudiment  determines  which  of  the  two  kinds  is  to 
appear. 

In  this  instance  we  see  a discontinuous  change  in 
conditions — the  change  from  water  to  air  as  a sur- 


CONCLUDING  CHAPTER 


318 

rounding  medium — giving  rise  to  a discontinuous 
change  in  structure.  Such  cases  are,  however,  com- 
paratively rare.  Much  more  usually  the  changes  in 
external  conditions  are  continuous,  as  changes  of 
altitude,  moisture,  or  chemical  composition  of  the  soil, 
and  so  on  ; and  the  changes  induced  by  them  in  the 
plant  are  similarly  of  a continuous  kind. 

In  most  animals  changes  in  external  circumstances 
have  a much  smaller  influence  on  the  form  and  struc- 
ture of  the  individual  than  is  the  case  with  plants.  In 
animals  considerable  modifications  are,  however, 
brought  about  by  exercise  and  the  use  of  different 
parts,  as  Lamarck  long  ago  observed.  But  these 
modifying  factors  usually  affect  all  the  members  of  a 
single  species  in  nearly  the  same  manner.  Neverthe- 
less, some  part  of  the  differences  between  individuals  in 
respect  of  strength  and  of  proportion,  and  possibly  also 
of  stature,  is  undoubtedly  associated  with  differences 
of  training  and  nutrition,  as  the  example  of  the  human 
race  is  sufficient  to  show.  Professor  Cope  has  pointed 
out  how  the  proper  development  of  such  structures  as 
the  joints  of  vertebrates  depends  to  a very  large 
extent  upon  exercise  ; and  the  effect  of  disuse  may  be 
practically  tested  by  anyone  whom  accident  obliges 
to  keep  a knee  or  other  joint  immovable  for  any  length 
of  time.  The  so-called  play  in  which  the  young  of 
many  animals  indulge — for  example,  lambs  and  kittens 
— must  have  a great  influence  upon  the  perfection  of 
their  locomotory  functions. 

We  can  now  see  more  clearly  the  reason  for  that 
great  instability  of  vegetative  type  which  sessile 


ACQUIRED  VARIATIONS 


319 


animals,  like  plants,  exhibit.  No  necessity  for  definite 
and  co-ordinated  movements  involving  their  whole 
structure  forces  the  development  of  these  animals 
along  certain  definite  paths.  External  circumstance 
is,  therefore,  free  to  mould  them  into  a host  of  slightly 
different  shapes.  And  thus  the  great  variability  of 
the  species  of  corals,  for  instance,  is  doubtless  deter- 
mined to  a large  extent  by  the  influence  of  different 
environmental  conditions. 

Strictly  speaking,  the  term  variability  ought  not  to 
be  applied  to  modifications  of  this  description.  It  wiU, 
perhaps,  be  most  convenient,  however,  to  distinguish 
true  variations — having  their  origin  in  differences 
among  the  germ-cells — as  genetic  variations,  contrasting 
them  with  the  acquired  variations  which  arise  during 
the  development  of  individuals. 

Enough  has  now  been  said  to  show  that  it  is  a very 
difficult  matter  to  distinguish  in  the  case  of  continuous 
variations  between  those  which  are  genetic  and  those 
which  are  acquired. 

It  is  easy  to  understand  how  acquired  variations 
come  to  be  continuous,  and  to  obey  the  law  of  normal 
variabihty.  We  saw  that  the  normal  distribution  of 
characters  was  induced  by  the  random  operation  of  a 
multitude  of  small  causes.  During  the  development 
of  the  individual  a great  number  of  different  external 
influences  come  into  play,  leading  to  slight  modifications 
of  every  part,  now  in  one  direction,  now  in  another. 
This  being  so,  we  may  be  quite  sure  that  a large  pro- 
portion of  the  normal  variability  which  any  species 
exhibits  is  acquired. 


320 


CONCLUDING  CHAPTER 


Now  we  saw  that  there  seems  to  be  good  evidence 
that  normal  or  continuous  variations  are  inherited. 
Logic  does  not,  however,  permit  us  to  make  the  step : 
Acquired  variations  are  continuous  variations ; con- 
tinuous variations  are  inherited ; therefore  acquired 
variations  are  inherited.  It  seems,  indeed,  to  be  this 
fallacy  which  has  led  to  the  long-continued  behef  in 
the  inheritance  of  acquired  characters  as  an  important 
factor  in  organic  evolution,  in  spite  of  so  many  argu- 
ments to  the  contrary. 

Formal  disproof  of  this  proposition  is  very  difficult, 
and  in  the  meantime  the  confusion  between  continuous 
acquired  variations  and  continuous  genetic  variations, 
which  is  always  present  in  practice,  constitutes  a very 
serious  drawback  to  the  biometric  method  of  research. 
At  present  Johannsen’s  explanation  of  these  phenomena 
seems  to  afford  so  much  the  simplest  solution  that 
we  may  once  more  repeat  his  statement  of  the  case, 
though  with  the  proviso  that  his  hypothesis  is  not 
universally  accepted. 

Johannsen  looks  upon  a population  which,  as  a 
whole,  exhibits  continuous  or  normal  variability,  as 
being  capable  of  analysis  into  a number  of  pure  hues. 
In  a single  pure  line  genetic  variabiUty  is  sensibly 
absent.  The  members  of  such  a pure  hne  exhibit, 
however,  very  considerable  acquired  variability,  so 
that  in  this  way  each  line  shows  a normal  variabihty 
of  its  own.  And  the  range  of  this  variabihty  may 
greatly  exceed  the  limits  which  separate  two  pure  hues 
from  one  another.  The  result  is  to  give  a completely 
blurred  picture  when  aU  the  hnes  are  looked  at  simul- 


PURE  LINES 


321 


taneously,  And  thus  the  normal  variability  of  the 
population  as  a whole  is  brought  about  by  the  com- 
bination of  these  two  separate  factors. 

This  statement  applies  to  the  case  of  an  organism 
in  which  self-fertilization  is  the  general  rule,  so  that  in 
this  way  the  separate  lines  are  kept  distinct.  Where 
cross-fertihzation  takes  place  between  the  members  of 
different  pure  lines  the  case  becomes  enormously  com- 
plicated, and  this  is  much  the  most  frequent  instance 
which  we  have  actually  to  deal  with.  It  has  been 
suggested  that  the  members  of  different  lines  when 
crossed  together  may  display  Mendelian  phenomena, 
but  the  existence  of  so  large  a proportion  of  acquired 
variability  renders  the  problem  of  analyzing  the  result 
almost  insuperable.  We  have  seen,  however,  that  the 
numerical  results  obtained  by  the  biometricians  do 
not  appear  to  be  inconsistent  with  the  existence  of 
Mendelian  inheritance  in  populations. 

We  find,  then,  that  the  questions  of  inheritance  of 
acquired  characters  and  of  evolution  by  the  aid  of  ^ 
continuous  genetic  variations  are  not  yet  absolutely 
settled.  Recent  discoveries  by  Winkler  and  Baur 
regarding  the  nature  of  so-called  ‘ graft-hybrids  ’ go 
far  to  prove  that  acquired  characters  cannot  be  in- 
herited in  plants.  The  classical  example  of  a ‘ graft- 
hybrid  ’ is  Cytisus  Adami,  which  was  produced  in  1825 
by  grafting  Cytisus  pmfureus  on  the  laburnum.  This 
plant,  which  is  more  or  less  intermediate  between  the 
two  parent  species,  has  been  reproduced  by  further 
grafting,  but  its  seeds  always  give  rise  to  plants  which 
are  indistinguishable  from  the  laburnum.  Adventitious 

21 


322 


CONCLUDING  CHAPTER 


shoots  exactly  resembling  those  of  one  or  other  of  the 
parents  often  appear  upon  the  ‘ hybrid  ’ plants. 

It  now  appears  that  in  Cytism  Adami  the  cells  of  the 
two  component  species  remain  perfectly  distinct,  and 
that  its  reproductive  cells  are  always  of  the  laburnum 
type.  In  spite  of  the  intimate  association  of  the  two 
groups  of  cells  which  build  up  a common  plant  body, 
the  cells  of  Cytisus  purpureus  are  unable  to  transmit 
any  hereditary  influence  to  the  cells  of  the  laburnum, 
and  these  give  rise  to  offspring  which  are  pure  laburnum. 
„The  epidermis  of  the  ‘ graft-hybrid  ’ is  said  to  consist 
wholly  of  cells  of  the  Cytisus  purpureus  type.  It  seems 
fair  to  argue  that  if  one  species  wrapped  in  the  epi- 
dermis of  another  receives  no  heritable  influence  what- 
ever from  its  living  integument,  it  is  in  the  highest 
degree  unlikely  that  the  germ  cells  will  be  able  to 
acquire  transmissible  modifications  from  an  environ- 
ment wholly  external  to  the  plant.  The  proof  may 
not  be  absolutely  conclusive,  but  when  it  is  combined 
with  all  the  other  evidence  pointing  in  the  same  direc- 
tion, we  think  that  the  inheritance  of  acquired  char- 
acters may  be  disregarded  as  a practical  factor  in 
evolution. 

Meanwhile  the  number  of  cases  in  which  discon- 
tinuity of  inheritance  can  be  shown  to  hold  good  is 
constantly  increasing,  and  the  analysis  of  some  cases 
of  supposed  continuous  variation  into  discontinuous 
Mcndelian  factors  has  already  been  made.  It  may 
be  safely  concluded  that  a very  large  part,  if  not  the 
whole,  of  evolution  has  taken  place  by  the  discon- 
tinuous method. 

New  little  species — Jordan’s  species — arise,  then, 


CONCLUSION 


323 


from  time  to  time,  each  at  a single  step,  from  pre- 
existing species.  Upon  the  material  thus  supplied 
natural  selection  operates  ; the  weaker  go  to  the  wall, 
the  stronger  survive.  This  is  also,  in  all  probability, 
the  way  in  which  adaptations  have  arisen.  Creatures 
which  came  into  existence  displaying  a particular  new 
structure,  which  happened  to  be  fitted  for  a particular 
new  function  or  suited  to  a particular  niche  in  Nature, 
survived  and  flourished  exceedingly.  Those  in  which 
undesirable  organs  appeared  perished  and  were  no 
more  seen.  To  take  Aristotle’s  example.  If  a man 
were  to  be  born  with  molars  in  front  and  incisors  at 
the  back  of  his  jaw  he  would  die — at  least,  in  the  days 
before  dentistry.  Having  his  teeth  in  the  positions 
in  which  they  actually  stand  (although  not  for  this 
reason  only),  he  survives  and  rules  the  world. 

After  all,  the  difference  between  the  point  of  view 
thus  briefly  indicated,  and  that  of  Darwin  as  expressed 
in  the  ‘ Origin  of  Species,’  is  only  one  of  detail — of 
detail  as  to  the  particular  sort  of  variations  by  which 
evolution  chiefly  proceeds.  Darwin’s  analogy  between 
the  origin  of  species  in  Nature  and  the  origin  of  races 
under  cultivation  may  be  repeated  with  emphasis, 
although  Huxley’s  famous  criticism,  to  the  effect  that 
races  which  are  sterile  together  have  not  arisen  in 
cultivation,  is  not  yet  completely  answered.  But 
this  renders  the  discontinuous  origin  of  such  sterility 
only  the  more  likely ; and  when  we  recall  the  Mendelian 
behaviour  of  such  characters  as  long  and  short  style  in 
the  primrose,  or  sterility  of  the  anthers  in  the  sweet- 
pea,  the  solution  of  the  problem  does  not  seem  very 
far  to  seek. 


324 


CONCLUDING  CHAPTER 


Let  us  see  how  the  principles  of  which  an  outline  has 
now  been  given  affect  the  human  race  itself.  The 
question  of  improving  the  human  stock  in  this  country 
has  lately  excited  a good  deal  of  attention.  But 
without  a scientific  knowledge  of  the  factors  upon 
which  improvement  and  degeneration  depend  the  dis- 
cussion is  not  likely  to  be  of  much  profit,  and  in  such 
a case  misdirected  energy  may  be  even  worse  than 
apathy.  Without  venturing  to  make  any  very  positive 
suggestions,  it  may  at  least  be  pointed  out  that  our 
present  practice  in  these  matters  is  in  almost  every 
case  the  very  worst  possible. 

Professor  Karl  Pearson  has  lately  shown  how  the 
low  birth-rate  of  the  professional  and  middle  classes — 
the  classes  amongst  which  the  intelligence  of  the 
nation  is  to  a large  extent  segregated — ^leads  to  the 
recruiting  of  these  classes  from  amongst  the  lower  and 
less  intelligent  strata  of  society.  In  other  words,  a 
steady  breeding  out  of  intelligence  is  taking  place. 
Recognising  that  intelligence  is  an  important  factor  in 
national  greatness,  we  proceed  to  remedy  this  defect  by 
endeavouring  to  reduce  the  infant  mortality  among  the 
less  desirable  classes,  and  by  offering  every  inducement 
to  the  production  of  large  families  by  the  lower  strata 
of  society ; indeed,  we  propose  to  remove  from  them 
all  responsibility  for  the  production  of  children,  and  to 
feed  and  house  the  latter  as  we  already  educate  them 
at  the  expense  of  the  State. 

The  principles  of  heredity  teach  us  that  education 
and  training,  however  beneficial  they  may  be  to  indi- 
viduals, have  no  material  effect  upon  the  stock  itself.  * 
If  they  have  any  effect  at  all,  this  is  undoubtedly 


CONCLUSION 


325 


unimportant  in  comparison  with  the  effect  which  would 
be  produced  by  the  selection  of  individuals  who 
exhibit  desirable  qualities.  The  demand  for  a higher 
birth-rate  ought  to  apply  strictly  to  desirables. 
Instead  of  this  the  cry  is  for  education  and  physical 
training,  processes  which  can  have  no  permanent 
beneficial  effect  upon  the  race. 

One  writer  who  holds  to  some  extent  the  attention 
of  the  intelligent  public  has  recognised  the  true  state 
of  affairs — I mean  Mr.  Bernard  Shaw.  Unfortunately 
the  public  does  not  take  Mr.  Bernard  Shaw  seriously, 
wherein,  when  I recall  Mr.  Shaw’s  published  views  on 
such  topics  as  vivisection  and  the  medical  profession, 
the  public  has  my  sympathy.  Nevertheless  I know  of 
no  better  expression  of  the  moral  to  be  dra^vn  from 
the  science  of  genetics  than  that  which  is  embodied  in 
the  following  passage : 

‘ I do  not  know  whether  you  have  any  illusions  left 
on  the  subject  of  education,  progress,  and  so  forth. 
I have  none.  Any  pamphleteer  can  show  the  way  to 
better  things,  but  when  there  is  no  will  there  is  no  way. 
My  nurse  was  fond  of  remarking  that  you  cannot  make 
a silk  purse  out  of  a sow’s  ear,  and  the  more  I see  of 
the  efforts  of  our  churches  and  universities  and  literary 
sages  to  raise  the  mass  above  its  own  level,  the  more 
convinced  I am  that  my  nurse  was  right.  Progress 
can  do  nothing  but  make  the  most  of  us  all  as  we  are, 
and  that  most  would  clearly  not  be  enough  even  if 
those  who  are  already  raised  out  of  the  lowest  abysses 
would  allow  the  others  a chance.  The  bubble  of 
heredity  has  been  pricked,  the  certainty  that  acquire- 
ments are  negligible  as  elements  in  practical  heredity 


326 


CONCLUDING  CHAPTER 


has  demolished  the  hopes  of  the  educationists  as  well 
as  the  terrors  of  the  degeneracy-mongers,  and  we  now 
know  that  there  is  no  hereditary  “ governing  class  ” any 
more  than  a hereditary  hooliganism.  We  must  either 
breed  political  capacity  or  be  ruined  by  democracy, 
which  was  forced  on  us  by  the  failure  of  the  older 
alternatives.  Yet  if  despotism  failed  only  for  want  of 
a capable  benevolent  despot,  what  chance  has  demo- 
cracy, which  requires  a whole  population  of  capable 
voters — that  is,  of  political  critics  who,  if  they  cannot 
govern  in  person  for  lack  of  spare  energy  or  specific 
talent  for  administration,  can  at  least  recognise  and 
appreciate  capacity  and  benevolence  in  others,  and 
so  govern  through  capably  benevolent  representatives  ? 
Where  are  such  voters  to  be  found  to-day  ? Nowhere. 
Promiscuous  breeding  has  produced  a weakness  of 
character  that  is  too  timid  to  face  the  full  stringency 
of  a thoroughly  competitive  struggle  for  existence,  and 
too  lazy  and  petty  to  organize  the  commonwealth 
co-operatively.  Being  cowards,  we  defeat  natural 
selection  under  cover  of  philanthropy  ; being  sluggards, 
we  neglect  artificial  selection  under  cover  of  delicacy 
and  morality.’* 

Mr.  Shaw  recognises,  however,  that  our  knowledge 
is  at  present  insufficient  to  prescribe  for  the  breeding 
of  a ‘ Superman,’  even  if  we  were  able  to  come  to  any 
agreement  as  to  what  qualities  are  the  most  desirable. 
Nevertheless  it  is  along  the  lines  which  we  have  en- 
deavoured to  indicate  that  such  knowledge  must  be 
sought  in  the  future. 

• ‘ Man  and  Superman,’  p.  xxiii. 


GLOSSARY 


{Many  technical  terms  not  included  in  this  glossary  are 
printed  in  italics  on  their  first  appearance  in  the  body  of  the 
book,  and  their  meaning  is  then  defined.  Such  definitions 
may  be  discovered  on  a reference  to  the  inde?f,'\ 

Adaptation.  — A teleological  explanation  of  the  corre- 
spondence often  shown  between  the  structure  and  habits  of 
a particular  creature  and  the  environment  in  which  the 
creature  lives. 

Albino. — An  animal  or  plant  characterized  by  the  absence 
of  colouring  matter  from  its  external  tissues. 

ALGiE. — A group  of  plants,  mostly  aquatic  and  of  re- 
latively simple  organization. 

Anther. — The  upper  part  of  a stamen,  containing  the 
pollen. 

Atom. — The  smallest  part  of  a chemical  element  which 
can  exist  as  such. 

Axil. — The  angle  enclosed  between  the  base  or  stalk  of 
a leaf  and  the  stem  upon  which  the  leaf  is  borne. 

Binomial  Nomenclature. — The  application  of  a double 
name  to  an  animal  or  plant,  the  first  name  being  that  of 
the  genus,  the  second  that  of  the  species. 

Biology. — The  science  of  the  phenomena  of  life. 

Biometry.  — The  application  of  statistical  methods  to 
biological  problems. 

Botany. — The  scientific  study  of  plants. 

Calyx. — The  outermost  whorl  of  floral  leaves,  which  in 
the  bud  usually  encloses  the  other  organs  of  the  flower. 

Character. — In  heredity,  a single  definable  attribute. 

Class. — One  of  the  larger  subdivisions  of  the  animal 
kingdom — ■e,g,,  mammals,  birds. 

327 


328 


GLOSSARY 


Composit-®.-“A  family  of  plants,  including  the  daisy, 
chrysanthemum,  and  many  others. 

Conjugation. — The  process  of  fusion  of  a pair  of  gametes. 

Corolla. — The  second  envelope  of  a flower,  consisting  of 
petals — leaf-like  organs — usually  brightly  coloured. 

Corpuscle. — A very  minute  particle. 

Cytology. — The  scientific  study  of  the  minute  con- 
stituent parts  of  organisms  by  the  aid  of  the  microscope. 

Denudation. — The  wearing  away  of  the  earth's  surface  by 
the  action  of  rain,  rivers,  etc. 

Differentiation.— The  separation  or  discrimination  of 
parts  which  were  previously  more  or  less  united  and  uniform. 

Embryo. — A young  plant  or  animal — usually  one  which  is 
still  contained  in  the  seed  or  the  womb. 

Embryology. — The  history  of  the  development  of  young 
plants  or  animals  from  the  egg. 

Environment. — Natural  surroundings. 

Evolution. — See  p.  22. 

Family. — A group  of  allied  genera,  as  the  family  of  apes 
{AnthropoidcB),  the  buttercup  family  (Ranunctilacece), 

Fauna. — The  sum  total  of  animals  inhabiting  a particular 
region. 

Fertilization. — The  union  of  male  and  female  repro- 
ductive cells  or  gametes. 

Florets. — The  separate  flowers  of  a crowded  inflorescence. 

Gametes. — Sexual  cells  which  unite  in  conjugation  or 
fertilization. 

Genus. — A group  of  allied  species. 

Geology. — The  study  of  the  earth's  crust. 

Geometric  Rate  of  Increase. — Progress  consisting  in 
successive  multiplications  of  the  preceding  number,  instead 
of  simply  in  additions  to  it. 

Germ-Cells. — See  Gametes. 

Herbalist. — One  who  collects  and  studies  herbs. 

Heredity. — The  transference  of  similar  characters  from 
one  generation  of  organisms  to  another,  a process  effected  by 
means  of  the  germ-cells  or  gametes. 


GLOSSARY 


329 


Igneous. — Produced  in  connection  with  great  heat. 

Inbreeding. — The  mating  together  of  near  relatives  for  a 
number  of  generations. 

Larva. — The  young  of  an  insect  after  it  has  emerged  from 
the  egg — e.g,,  a caterpillar. 

Mantidae. — A group  of  predatory  insects. 

Maxillary. — Connected  with  the  mouth  parts. 

Morphology. — The  study  of  form  and  structure. 

Mutation. — The  sudden  origin  of  a new  species  at  a single 
step. 

Organism. — A living  creature. 

Ornithologist. — A student  of  birds. 

Ovary. — In  animals  the  organ  which  produces  ova.  In 
plants  the  organ  which  contains  the  ovules. 

Ovum. — The  female  gamete. 

Ovule. — The  structure  surrounding  the  spore  which  gives 
rise  to  the  female  gamete  or  ovum  in  the  higher  plants. 

Petal. — One  of  the  (usually)  coloured  leaves  composing 
the  corolla. 

Petaloid. — Resembling  the  corolla,  usually  in  the  circum- 
stance of  being  coloured. 

Physiology. — The  study  of  the  functions  of  organisms. 

Pin-eyed. — Having  the  stigma  on  a level  with  the  throat 
of  the  corolla,  and  the  anthers  lower  down,  enclosed  within 
the  tube. 

Pistil. — The  central  organ  of  a flower,  which  contains  the 
ovules,  and  ultimately  becomes  the  fruit,  or  the  chief  part 
of  it. 

Pollen. — Those  spores  of  the  flowering  plants  which 
produce  the  male  gametes. 

Pollination. — The  transference  of  pollen  to  the  stigma  of 
a plant. 

Primary,  Secondary,  and  Tertiary  Epochs. — The  three 
great  divisions  of  geological  time  during  which  the  known 
fossiliferous  strata  were  deposited. 

Radical  Leaves. — Leaves  arising  immediately  from  the 
root-stock  in  the  form  of  a rosette. 


330 


GLOSSARY 


Reversion. — The  reappearance  in  the  offspring  of  a 
character  proper  to  a more  or  less  remote  ancestor,  and  not 
exhibited  by  the  immediate  parents. 

Rotifers. — A kind  of  minute  aquatic  animals. 

Segment. — One  of  a series  of  more  or  less  similar  trans- 
verse divisions. 

Sessile. — Fixed  and  stationary,  but  (in  the  strict  sense) 
without  a stalk. 

Somatic. — Belonging  to  the  body  of  a zygote. 

Species,  Linn^an.  — A group  of  organisms  of  closely 
similar  appearance. 

Species,  Jordan’s. — A group  of  organisms  believed  to  have 
arisen  by  a mutation.  (Jordan  himself  did  not,  however, 
suppose  so.) 

Sport. — A marked  mutation — often  one  occurring  under 
domestication. 

Stamens. — The  organs  of  a flower  which  bear  the  pollen. 

Standard. — The  large,  upright  petal  at  the  back  of  such 
a flower  as  that  of  the  sweet-pea. 

Stigma. — The  uppermost  part  of  the  pistil,  upon  which  the 
pollen  is  received. 

Stratum. — A layer. 

Style. — A stalk  connecting  the  stigma  with  the  ovary — 
part  of  the  pistil. 

Testa. — The  skin  or  coat  covering  a seed. 

Thrum-eyed. — Having  the  anthers  situated  at  the  throat 
of  the  corolla,  and  the  stigma  lower  down,  enclosed  in  the 
tube. 

Tube. — The  basal  tubular  portion  of  a corolla  in  which 
the  separate  petals  are  closely  fused  together,  as  is  the  case 
with  that  of  the  primrose. 

Unicellular. — Consisting  of  a single  cell. 

Variation,  Continuous. — See  Chapter  IV. 

Variation,  Discontinuous. — See  Chapter  V, 

Wings. — The  lateral  petals  of  a pea-flower. 

Zoology. — The  scientific  study  of  animals. 

Zygote. — The  organism  produced  by  the  fusion  of  a pair  of 
gametes. 


INDEX 


Abraxas  grossulariata,  278 
Acquired  characters,  inheritance 
of,  39,  65  et  seq.,  123,  155 
variations,  319 
Adaptation,  et  seq,,  325 
^Albino,  132,  327 
Algae,  306,  327 

Allelomorphs,  185,  195,  203  et 
seq,,  302 
Allogene,  229 
Allozygote,  229 
Amoeba,  240 

Ancestral  heredity,  law  of,  34, 
1 13  et  seq.,  228 
Andalusian  fowl,  192 
Andreae,  Dr.,  58 
Argus  pheasant,  63 
Aristotle,  124,  323 
Array,  104 
Ascaris,  251 

Atom,  chemical,  19,  24,  127 

Bachmann,  7 
Barley,  117,  136 
Basset  hound,  107 
Bateson,  Prof.  W.,  viii,  17,  60, 
61,66,124,  128  etseq.,  133,134, 
192,  200,  215,  238,  312  et  seq. 
Baur,  321 
Beans,  kidney,  117 
Beetles,  Chrysomelid,  155 
Bentham  and  Hooker,  8 
Between-race,  148 
Biffen,  Prof.  R.  H.,  233  et  seq. 
Biometry,  18,  80  et  seq.,  160, 
227  et  seq. 

Birth-rate,  290  et  seq. 
Blaringhem,  156 


Blind,  tactual  discrimination  in 
the,  69 
Bonnier,  317 
Boveri,  252  et  seq.,  283 
Browne,  Sir  Thomas,  124 
Burbank,  Luther,  44 
Busbey,  Hamilton,  137 
Butler,  Samuel,  79 
Butterflies,  215,  57 
tropical,  56 

Casiiarina  indica,  59 
Cat,  70,  215 

Cell,  72  et  seq.,  239  et  seq.,  272, 
274 

Cereals,  233  et  seq. 

Cetacea,  52 

Characters,  acquired,  39,  65  ei 
seq.,  123,  155 
mental,  no  et  seq. 
variations,  319 
Chromatin,  248 
Chromomeres,  268 
Chromosomes,  248  et  seq.,  272 
et  seq.,  306 
Classes,  88,  97 
Clover,  five-leaved,  150 
Coat  colour  of  horses,  109,  no, 
ii3»  215 

Coefficient  of  variability,  99 
Coins,  spinning  of,  91  et  seq. 
Colour,  inheritance  of,  13 1,  213 
et  seq. 

of  animals,  56 
Colour-blindness,  277 
Comparison  of  species,  12 
Compositors,  tactual  discrim- 
ination in,  69 


331 


332 


INDEX 


Conscientiousness,  iii 
Controversial  methods,  3 
Co-ordinated  structures,  69 
variability,  123 
Cope,  Professor,  318 
Correlated  variability,  106 
Correlation,  loi  et  seq, 
coefficients,  106  et  seq, 
Correns,  19,  190,  301 
Coupling,  197  et  seq. 

Cowslip,  8 
Criminals,  297 

Crustaceans,  loss  of  limb  by, 
62 

Cryptomerism,  203 
Cuenot,  206  et  seq. 

Curve,  normal,  96 
Cytisus  Adaini,  321 
purpureus,  321  ^ 

Cytology,  239  et  seq.,  305 

Dalton,  19,  175 

Darwin,  Charles,  2,  3,  16,  33, 
38,  40,  41,  42  et  seq.,  54, 
123,  124,  139,  160,  190, 
215,  280,  312,  323 
Erasmus,  29 
Sir  Francis,  xi 
Sir  George,  28 
Daughter  cell,  242 
Davenport,  99 
Degressive  mutation,  153 
De  Maillet,  30 
Determinants,  280 
Deviation,  98,  125^ 

De  Vries,  17,  19,  75,  76,  83,  12 1, 
135.  139.  ef  ^eq.,  158,  190,  216, 
218,  267  et  seq.,  313  et  seq. 
Di-hybrids,  185  et  seq. 
Discontinuous  variation,  128  et 
seq.,  311  et  seq. 
Domestication,  changes  under, 
34  et  seq. 

Dominance,  192 
Dominant  characters,  179 
Doncaster,  Dr.  L.,  vfi,  21,  27S, 
283 

Double  flowers,  134 
Drosophila,  200,  270,  277,  303 


Druery,  C.  T.,  36 
Dyer,  Sir  W.  Thiselton,  35 

Elephant,  46 

Elk,  69,  71 

Embryo,  177 

Embryology,  31,  243 

Emily  Henderson  (sweet  pea),  209 

Empedocles,  29 

Endosperm,  177 

Error,  probable,  99 

Eugenics,  20,  284  et  seq. 

Education  Society,  298 
Evening  primrose,  83 
Evolution,  22  et  seq.,  122,  300, 

307 

parallel,  57,  59 

Eye,  colour  of  human,  82,  no 
Ferns,  36,  307 

Fertilization,  162,  243,  244,  246, 
3I3»  321 
normal,  219 
Flat  fishes,  38 

Flies,  flovrer-frequenting,  58 
Fluctuating  variations,  75 
Fly,  fruit-,  200,  270,  277,  303 
Formula,  Mendelian,  183  et  seq. 
Fowls,  192,  215 
Franklin,  45 

Fraternal  correlation,  106  et  seq, 
Frequency,  98 

Gaertner,  C.  F.  v.,  170 
Galton,  Sir  Francis,  80,  89,  107  * 
113,  125  et  seq.,  284  et  seq. 
Gametes,  178,  244,  245,  306 
Gardiner,  Professor  Stanley,  xi 
Gates,  R.  R.,  146,  272 
Geikie,  28 
Genetics,  285,  300 
Genetic  variations,  319 
Geographical  distribution,  33 
Geological  succession,  33 
time,  27,  315 

Germ-cells,  72  et  seq,,  178,  244, 
245,  282,  306 

Germ-plasm  theory,  279  et  seq. 
Giraffe,  37 


INDEX 


333 


Goethe,  30,  175 
Goss,  John,  169 
Gradation  of  organisms,  31 
Greeks,  29 
Greenland  whale,  52 
Gregory,  R.  P.,  xi,  215,  272 
Greyhound,  coat  colour  of,  no 
Guinea-pigs,  214 
Gulick,  50 

Habit  of  plants,  316 
Half-race,  148 
Harwood,  44 
Hawaiian  Islands,  50 
Helium,  23 

Herbert,  Hon.  and  Rev.  W.,  169 
Hermaphrodite,  310 
Heredity,  i,  4,  5,  loi,  112  et 
seq,,  174  et  seq.,  272  et  seq,, 

324 

Heron,  294 

Heterotropic  chromosome,  273 
et  seq. 

Heterozygote,  184  et  seq.,  301  et 
seq. 

Heterozygote  forms,'  192,  302 
Hieracium,  190 
Hippocampus,  55 
Homoeosis,  133,  134 
Homozygote,  185,  192,  301  et 
seq. 

Horns,  63,  69 

Horse,  American  trotting-,  137  - 
et  seq. 

coat  colour  in,  109,  215 
extinct,  33 
Hume,  David,  29 
Hurst,  215 
Hutton,  26 

Huxley,  20,  124,  294,  323 
Hybrid,  159  et  seq. 
false,  172 
graft,  321 
Hypostasis,  213 

Idiochromosomes,  270,  273  seq* 
Ids,  281 

Immunity  to  rust,  237 
Indian  corm  1^6,  177  seq. 


Inheritance,  bisexual,  118 

of  acquired  characters,  65 
et  seq.,  155,  321,  324,  325 
of  colour,  131,  213  et  seq. 
of  colour-blindness,  277 
of  mental  characters,  no  et 
seq. 

of  physical  characters,  103 
et  seq. 

Irish  elk,  69 
Isolation,  50 

Japan,  58 
Jennings,  116 

Johannsen,  Professor,  116  et 
seq.,  122,  320 
Jordan,  8,  153 
Jordan’s  species,  9,  153,  322 

Kelvin,  Lord,  27 
Knight,  T.  A.,  166  et  seq. 
Knight-Darwin  law,  168 
Kolreuter,  J.  G.,  161  et  seq. 

Laburnum,  321 

Lamarck,  15  et  seq.,  30,  36  ct 
seq.,  65,  123,  318 
Laplace,  24 

Latent  characters,  153,  203 
Leaf  insects,  55 
Leicestershire  sheep,  137 
Line,  pure,  117  et  seq.,  320 
.Linkage,  202 
Linnaeus,  7 e^  seq. 

Lock,  R.  H.,  xiii  et  seq.,  196 

Lotsy,  Dr.,  xi 

Lyell,  Sir  Charles,  25,  41 

MacDougal,  Prof.,  146,  156,  296 

Maize,  156,  177  et  seq.,  197 

Malthus,  45,  46 

Manitoba  Hard  (wheat),  235 

Mantidae,  55 

Maple  pea,  205 

Marriage  of  unfit,  294  et  seq. 

State  aid  for,  296 
Masked  characters,  203  et  seq. 
Masters,  133 
Matthews,  Patrick,  42 


334 


INDEX 


Mean  of  normal  curve,  96 
Medium  of  normal  cun^e,  96 
Mendel,  Gregor,  19,  86,  113, 
118,  174  et  seq,y  264  et  seq., 
300  et  seq. 

Mendel’s  law,  189,  300  et  seq. 
Mental  characters,  resemblance 
in,  no 
Merism,  129 
M eristic  variations,  130 
Metamorphy,  133 
Mice,  207,  214 

Michigan  bronze  (wheat),  237 
Mid-race,  148  et  seq. 

Millardet,  172,  216 
Mimicry,  54  et  seq. 

Mirabilis  jalapa,  165 
longijiora,  165 
Mitosis,  248 
Mivart,  38 

Mode  of  normal  curve,  96 
Mongrel,  160 
Mono-hybrid,  183  et  seq. 
Morgan,  Prof.  T.  H.,  61  et  seq., 
79,  269,  303 
Morphology,  32,  329 
Moth,  Currant,  278 
Mother-cell,  242 
Mutation,  36,  121  et  seq., 
artificial,  156 
atavistic,  154 
causes  of,  155 
degressive,  153 
progressive,  153 
retrogressive,  154 
Mutationists,  17 

Nageli,  190,  280 

Natural  selection,  42  et  seq.,  48, 

121,  135 

Naudin,  C.,  170,  220 
Nebular  hypothesis,  24 
Neo-Darwinians,  3,  16 
Neo-Lamarckian,  3 
Newton,  Sir  Isaac,  175 
Nicotiana  paniculata,  163 
glutinosa,  166 
rustica,  163 
tdbacum,  166 


Normal  variations,  82.  et  seq. 
114,  116 

curve,  95  et  seq. 

Nuclear  division,  248  et  seq. 

membrane,  246 
Nucleus,  177,  239,  246  et  seq. 
Nutrition,  76,  155 

Oenothera  alhida,  143 
biennis,  147 
gigas,  143 

Lamar chiana,  142  et  seq. 
3i3»  314 

Iceta,  219 
lata,  143 
muricata,  147 
nanella,  144,  147 
velutina,  219 

‘ Origin  of  Species,’  124,  323 
Ovum,  72,  177,  24s,  308 
Oxlip,  8 

Painted  Lady  (sweet  pea),  210 
Papaver  rhoeas,  140 
Parental  correlation,  loi  et  seq 
Partial  gametic  coupling,  200 
Pea,  garden,  84,  166  et  seq.,  190 
198,  204 

sweet,  209  et  seq. 

Peacock,  63,  132 
Pearson,  Karl,  77,  80  et  seq. 
108,  no,  116,  136,  229,  287 

324 

Phasmidae,  55 
Phyllopteryx,  55 
Physical  science,  4 
Picotee  (sweet  pea),  210 
Pigeon,  34,  45,  215 
Plastids,  240 
Pleurococcus,  241 
Polish  wheat,  234 
Polygon,  Gabon’s,  126 
Population,  117,  118,  320 
selection  in,  119 
Poulton,  Professor,  57 
Primrose,  8,  215 
evening,  83 
pin-eyed,  215 
thrum-eyed,  215 


INDEX 


335 


Primula  pyramidalis , 194 
sinensis,  15 1,  194,  272 
stellata,  194 

Probability,  theory  of,  91 
Progressive  mutation,  153 
Pronuha,  54 

Protective  resemblance,  54  et 
seq. 

Protenor  helfragi,  272 
Prothallus,  308 
Protogene,  229 
Protoplasm,  239 
Protozoa,  116 
Protozygote,  229 
Puccinia  glumarum,  237 
Punnett,  Prof.  R.  C.,  47,  192, 
200,  238 

Pure  lines,  116  et  seq,,  320  et 
seq. 

.selection  in,  119 
Purple  Invincible  (sweet  pea), 
210 

Quartile,  97 
Quetelet,  80,  86,  93 

Rabbit,  214 
Radium,  23 
Raimannia,  156 
Ranunculus,  7 

water,  150,  317 
Recessive  characters,  179 
Red  Fife  (wheat),  235 
Reducing  division,  258  et  seq. 
Regeneration,  61  et  seq. 
Regression,  107 

Resemblance,  protective,  54  et 
seq. 

Retrogressive  mutation,  154 
Reversion  on  crossing,  202  ct 
seq,,  213 

Rivet  wheat,  234 
Romanes,  G.  J.,  41,  79 
Rosenberg,  254 
Rotifers,  47,  330 
Rough  Chaff  (wheat),  235 
Rust  immunity,  237 

Saunders,  Miss,  209,  214 
Saxifraga  hypnoides,  59 


Schuster,  E.,  299 
Sea-horse,  55 
Sea-urchin,  256 
Sedgwick,  A.,  75 
Selection,  artificial,  44,  114  et 
seq,,  135,  141 

natural,  42  et  seq,,  48  ct 
seq,,  76,  141,  157,  295 
sexual,  62  et  seq. 

Selective  fertilization,  209,  274 
Sex,  221,  272  et  seq, 

Shaw,  G.  B.,  325 

Sheep,  Leicestershire,  137,  215 

Shirley  poppies,  140 

Shull,  Dr.  G.  H.,  213 

Snails,  215 

Snakes,  37 

Somatic  cells,  74,  272,  274 
Species,  idea  of,  6 

Jordan’s,  9,  153,  322 
Linnaean,  7 et  seq. 

Origin  of,  5,  124,  147  ci 
seq,,  323 

Spencer,  Herbert,  23,  40,  47,  65 
et  seq,,  71,  122 
Spermatozoa,  245,  310 
Spindle,  nuclear,  250 
Spores,  257,  307 
Standard  deviation,  98 
Statistics,  12,  81 
collection  of,  86 
Stature,  90,  106 
Sterility,  323 
.Stick  insects,  55 
Stocks,  214 
Strawberries,  172 
Strength  of  pull,  89 
of  wheat  flour,  235 
Structural  characters,  inherit- 
ance of,  185 

Substantive  variation,  13 1 
Subtypes,  126,  127 
Sugar-beet  industr}^,  135 
Superman,  326 
Survival  of  the  fittest,  47 
of  useless  structures,  157 
Swamping  effect  of  crossing, 
221 

Sweet  peas,  209  et  seq. 


336 


INDEX 


\ Symmetry,  129 
SyrphidcB,  58 

Tactual  discrimination,  66  et 
seq, 

.Tetrads,  259 

Thomson,  Sir  J.  J.,  24,  127 
Throw-back,  154 
Tinged-white  sweet  pea,  21 1 
Tower,  W.  L.,  155 
Treviranus,  30 
Trifoliiim  pratense,  150 
Tschermak,  19,  190,  203 
Types,  primary  and  subordinate, 
125 

Twins,  287  et  seq. 

Unicellular  animals,  240,  330 
Unit  characters,  176,  220,  315 
Uranium,  23 
Usher,  Bishop,  27 

Variability,  acquired,  319 
correlated,  106 
genetic,  319 
coefficient,  99 
Variate,  97 
Variation,  14 

continuous,  14,  43,  76  et 
seq.,  82,  155,  316  et  seq. 


\ ansition— continued : 

discontinuous,  14,  43,  66, 
I35j  316  et  seq. 
meristic,  130 
negative,  14 1 

similar  and  simultaneous, 
132 

substantive,  130,  13 1 

Wading  birds,  38 

Wallace,  A.  R.,  i6,  35,  43,  65, 

88,  157 

Webb,  Sidney,  295 
Weismann,  16,  51,  52,  65,  72  et 
seq,,  157,  279  et  seq. 

Wells,  Dr.  W.  C.,  42 
Werner,  25 
Whale,  52 

Wheat,  136,  233  et  seq, 
Whetham,  W.  C.  D.,  23,  41 
Wilks,  Rev.  W.,  140 
Willey,  A.,  79 

Wilson,  Prof.  E.  B.,  74,  272  et 
seq, 

Winkler,  321 

X-generation,  306 

Yucca  plant,  54 
Yule,  G.  U.,  227  et  seq» 

Zygote,  185,  245 


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